Add pin mapping code.

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.