Technically I updated to a development version somewhere between 17.6.3
and 17.6.4, but I don't doubt its stability.

This is a good update because I'd like users of the library to be
confident that it works with the latest version of ChibiOS.
Interestingly, none of the changes I made were in the library code, so
it did before, but now I know for sure.

I'm unclear as to whether I can now get rid of the INT_N thread in favor
of a pin interrupt.  I'll have to see if I can, or if that will have to
wait for still more ChibiOS updates.

After much poking around, I finally figured out how to make this work!
The problem was that the ChibiOS initialization code had disabled
interrupts before the __early_init function was run, making the
bootloader not work despite my best intentions.  This was fixed by
enabling interrupts just before jumping to the bootloader.  This in turn
required changing to GNU-flavored C11 to include an inline assembly
instruction.

The code compiles again, and it seems to work as it did before.  There's
still a lot to do (we still have a mess of globals), but the PE
correctly calls the new DPM functions via the list of callbacks in the
configuration object.

There are now a few extra lines in the Makefile that make the PD Buddy
firmware library's makefile get included, and the variables it defines
get used.  There's no code to compile or include yet though.

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.

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.

There are still a few things that the standard says we Shall do and we
don't, but it Works For Me™.  I haven't implemented anything with
regards to GiveBack support, but that doesn't matter just yet.  Our
handling of VDMs isn't quite right either.  Anyway, it successfully
negotiates with so-called Split PDO power supplies, which is more than I
can say about some commercial products.

Based on the RT-STM32F072-DISCOVERY demo from ChibiOS/demos/STM32.  The
board definition was made with the Eclipse plugin, and is based on the
ST_STM32F072B_DISCOVERY board definition from ChibiOS/os/hal/boards.