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.

Now we use more consistent language to refer to the stored configuration
(which is in flash) and the configuration buffer (RAM).

PD uses odd units for voltages and currents.  Power too, but we don't
work with power (yet).  In the shell, we have to convert to and from
these odd units.  We had been doing this in-place every time, which was
a bit crumby.  Now it's done with macros in pd.h, making prettier and
more maintainable code.

The configuration interface is done, so we shouldn't have any comments
that say otherwise.  Also, since the shell never exits, it's silly to
put the code that runs it in a loop.

The command-line interface now shows license information if you type the
"license" command.  Now users can be easily made aware of their
freedoms.

The GUI will have a button to identify a particular PD Buddy Sink, in
case several are connected to the computer at once.  The button will
cause the LED to blink quickly for a couple seconds, then go back to
normal.  To allow this, I added a new LED blink mode (FAST_BLINK_SLOW)
that causes 8 fast blinks, then returns to SLOW_BLINK mode.  A new
"identify" command triggers this LED mode.

The command shell is based on the ChibiOS-provided one, but modified to
not run in a new thread and generally be a bit simpler, while also
having PD Buddy branding.  The storage functions work with the STM32
flash registers directly, taking over the last page of flash.  A new
linker script is used to ensure that program code can't end up using
that page.  The DPM has been updated to use the saved settings rather
than hardcoded ones.  Now what's needed is real commands for editing the
configuration.

Added commands to edit configuration

We now have commands to load the current settings into RAM, edit the
settings in RAM, and write them back to flash.  The only settings
currently supported are current and fixed voltage, because the rest of
the code doesn't support GiveBack or variable/battery PDOs yet.

Short help messages were added for each command.  Nothing too exciting,
but it should be a great help for someone poking around in the PD Buddy
Sink configuration shell for the first time.

Wrote new usage information for the firmware

Now that we have a command shell, the README should indicate how to use
it.