Rather than having the autobuild know about the particular variants, have
the mpconfigboard.mk describe them and make autobuild discover them
automatically.
Adds a "query-variants" target to stm32/Makefile to allow the set of
possible variants to be queried.
Removes pybv3 from the autobuild as this isn't use by the downloads page.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>
492ba5eaf2