This commit also introduces board directories and moves board
specific config into the appropriate board directory.
boards/stm32f4xx-af.csv was extracted from the STM32F4xx datasheet
and hand-tweaked.
make-pins.py takes boards/stm32f4xx-af.csv, boards/stm32f4xx-prefix.c,
and boards/BOARD-NAME/pins.csv as input and generates the file
build/pins_BOARD_NAME.c
The generated pin file for PYBOARD4 looks like this:
https://gist.github.com/dhylands/9063231
The generated pins file includes all of the supported alternate
functions, and includes upsupported alternate functions as comments.
See the commnet block at the top of stm/pin_map.c for details on
how to use the pin mapper.
I also went ahead and modified stm/gpio.c to use the pin mapper.
Note: none of these functions are used by STM port, so they're provided
to have parity with pre gc refactor status quo, and in case they will be
needed in the future by some extension writers.
