add basic adc module (using sdadc only atm)

this has a blocking adc read that does a single conversion and that is
it atm.

it is only designed to work with sdadc with its excellent 16 bit
resolution etc as that is what I need, but it could be extended later.
fix add_header.py to find crc import from rel path

before it would import crc from anywhere in the path, now it is relative
to the project so you know your using the libcrc and crc.py from the
current checkout
Merge branch 'master' into stm32f373
clean up bootstrap linker script
clean up python crc libs properly

we were leaving crc.pyc even after a make clean