Firmware 1.2.0 will have more configuration flags than previous
versions.  This commit adds macros for those flags.  There's also a
field added to the flags halfword that says whether a current, power, or
resistance is configured, so this commit makes the clear_flags command
not modify that field.

Now instead of checking the Var/Bat flag, it checks if either end of the
range is non-zero.

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.

The "storage" source code files contained lots of stuff with "config" in
the name, but nothing named "storage".  This was unlike every other part
of the firmware, so I renamed the files to "config".

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.