Initial import of the stm32f3discovery board

- import of board makefiles
- import of board.h
--> board.c is still missing due to licensing problems

The board configuration in board.h is using the low-level periphial driver as proposed in PRs #611 to #616.

In this state the board is not complete yet, I have further commits pending and will append them once we solved the licensing issue with some ST-lib start-up code...