Each field's name and meaning is listed now, and the unsupported types
are no longer listed as if they are supported.  As always, I'd like to
support variable and battery PDOs, but I need hardware that sends them
to add such support.

Since the spec says we Shall request no more power than we actually
need, we were violating the spec whenever the output was disabled in
Setup mode.  This commit fixes that, requesting low current at 5 V when
the output is disabled.

We do play a little fast and loose with the MAX_CURRENT field, but
that's hard to avoid in a device like this.  Well, we could always set
it to 5 A in Setup mode to be safe, but that's not necessarily right
either.  At least it's still honest in Sink mode, which I consider more
important anyway.

We don't need to update the source's capabilities when setting our
output state.  Now we just send a new Request.  For now, that request is
redundant, but soon I want to make the Sink request less current when
the output is disabled.  Whether the voltage should be the same or
always vSafe5V in that case, I haven't decided yet.  There are
advantages to each, but the difference is so minor in terms of using the
device that I don't see a reason to let the user choose, which of course
puts the decision on me.

It was never really necessary to update the source capabilities before
making sending a Request message.  It was convenient though, since we
didn't store the previous Source_Capabilities message.  Now we do, so
it's possible to make a new Request based on that.

This commit makes the Sink do just that.  It doesn't follow the spec's
state diagram for this, but it's a minor change that works better with
this firmware's design, so I don't mind.  There's a comment explaining
that too, so future folks won't be confused by it either.

The `PDO n:` and `type: blah` lines have now been combined into a single
`PDO n: blah` line.  This saves one line per PDO, and still makes plenty
sense.

When we were getting power from Type-C Current advertisements, we used
to not update the location of the configuration object.  This was done
to save time between polling the Type-C Current, and was not a problem
until we started trying to do Power Delivery in Setup mode.  As the
comment correctly said before, when we were trying to get power, the
location of the configuration object wouldn't change, so there was no
need to find its location more than once.

Now that the location of the configuration can change while we're
getting power, we need to check if that's happened.  This can be easily
done by checking the configuration object's status.  If the status is
not VALID, we get the location of the new configuration object.

It prints the most recently advertised PDOs in a reasonable format.  The
types it doesn't know about are printed as hex so that someone
knowledgeable can still get the information they need.  For now, it only
knows about fixed PDOs, so variable and battery PDOs will be shown as
hex.

Since this means we remember the most recent Source_Capabilities, it's
now becoming possible to send a new Request without sending a
Get_Source_Cap message first.  That's not actually done yet, but I'd
like to do it that way.  I'd also at some point like to make a
lower-current Request when the PD Buddy Sink's output is disabled, but
again, that's still to come.

Still preliminary, but the general idea is that each PDO will be printed
similarly to how a configuration object is printed, but numbered and
indented to make a nice human- and machine-readable list.

Just as specified last night.  This represents the completion of a big
part of the new interactive Power Delivery features of the shell.
Next up: printing the source's advertised PDOs.

The new behavior of `write` has been documented, as well as a high-level
description of the new Setup mode behavior with Power Delivery
communications.

The new `output` command now has documented specifications.  Not a
single line of code has been written for it though, but this is isn't a
release so you shouldn't take everything the documentation file says as
true.

When running in Setup mode, the Sink is capable of USB communications.
Now it's honest about that in the Request and Sink_Capabilities
messages.

The PD Buddy Sink now requests new power after configuration is written.
This means that in Setup mode, the voltage and current can be
re-negotiated on the fly.  Cool, huh?

It's still impossible to turn the output on and off from the shell.  New
commands will allow that soon enough, though.  Also, I'm seeing some
weird behavior when switching to/from 5 V (power shuts off entirely
sometimes), but I suspect that's a quirk of the source I'm using (Asus
USB 3.1 UPD Panel) and not the PD Buddy Sink itself.  I plan to make or
buy a USB power/data splitter to verify this.

The DPM used to always set the LED to indicate the PD status.  This
caused fighting when the PD threads were run simultaneously with the
shell, making the LED show different things depending on what commands
the user ran.  Not cool!

This commit adds a bool pdb_dpm_led_pd_status, which prevents the DPM
from setting the LED when set to false.  This commit also sets it to
false before starting the PD threads in Setup mode, allowing the shell
to be in full control of the LED.  Right on!

A good first step towards the upcoming 1.1 release, this commit runs the
Power Delivery threads in Setup mode.  This required a slight change to
the shell to make it non-blocking, as otherwise the PD threads would
never get to run.

There's still a lot to do!  The shell and the PD threads fight over
control of the LED in Setup mode.  There's no way to make the PD threads
re-negotiate the required power.  There's also no way to turn the output
on or off from the shell.  None of these changes should be too major,
but together they'll be pretty cool when they're all done.

Starting from v0.3, the PD Buddy Sink is sold without SW1 installed.  It
might be frustrating to see the README mention setting the switch only
to find that there isn't actually a switch to set.

Now the README mentions that there might not be a switch, and what to do
if there isn't.  Also, it's more specific about what position for the
switch makes the device enter DFU mode.

The shell main loop function and text input function were dreadfully
devoid of comments.  This simply could not continue, so I added a
sprinkling of comments to each, as well as to important sections of the
file.  Further, I changed the special character values to look like
characters rather than numbers, which should aid in understanding.

Now the error behavior for set_v and set_i is well-defined by the
documentation: they print nothing on success and an error message on
failure.  The range of valid values is now [0, 20000] for set_v and
[0, 5000] for set_i, matching the voltages and currents that can be
provided by USB Power Delivery.

Bumped version number to 1.0.1, reflecting this bugfix.

The iSerial descriptor now holds the firmware version number.  This is
1.0.0 since I don't expect any major API changes any time soon.  The USB
descriptors have been documented in docs/console_config.md so there
will be no questions as to how to identify the devices and what their
"serial numbers" mean.

The VID:PID pair 1209:9DB5 is now officially assigned for the PD Buddy
Sink.  Thanks, Arachnid!  Now the firmware uses its proper PID instead
of the testing one.

The most useful thing from the stack of scratch paper I made while
designing the firmware.  Now it's immortalized as an SVG.

As the name suggests, it clears all the flags in the configuration
buffer.  This provides a way to easily set all the flags to a known
state, which is nice for implementing libraries that configure PD Buddy
Sink devices.

There's not really any harm in writing a configuration with zero voltage
or current (and it was possible all along anyway), so why fight it?

Since the firmware is under 32 KiB, I figured there's not much reason to
use a microcontroller with 128 KiB of flash.  The new boards use
STM32F072x8 chips, which only have 64 KiB of flash.  This allows for the
firmware to grow, while not being quite so ridiculous as before.

This commit sets the Makefile to put the configuration at the end of the
64 KiB flash chip, and to use a new linker script for the same chip.

That's the name of the board now, so the configuration should match.

It's more obvious what is meant by that macro than by just the number
100.

It's very important to document the format in which configuration is
printed, since being able to reliably read the configuration is
essential to implementing configuration GUIs.  Now it's documented, and
the format is considered stable.

Now there's a docs/ directory for storing our documentation in!

The manufacturer should be PD Buddy, and the product should be Sink.
And so it is.

Now we correctly set the GiveBack flag and minimum current fields in the
Request message when GiveBack is enabled.  Also when GiveBack is
enabled, we correctly handle GotoMin messages by lowering our power
consumption, completing negotiation, and Requesting our normal power
periodically until the power supply Accepts our Request.

At least, I believe it's correct.  I don't have any hardware that sends
GotoMin messages, so I can't test it other than that it doesn't break
anything with the GiveBack flag set to 1 or 0 when the source doesn't
ask to reduce power.

As the name says, it toggles the GiveBack flag in the configuration
buffer.  The rest of the firmware doesn't use the flag for anything yet,
but it is correctly saved in flash.

We used to free the Request we made after transmitting it.  However, for
GiveBack to work properly, we need to be able to make that request
again.  To do it without having to request the Source_Capabilities
again, it makes sense to keep the Request message in memory.  We have
more messages in the pool than we need anyway.