board/cpu: Added support for the stm32f3discovery board and stm32f3 cpu

To complete the STM32FxDiscovery family, now also basic support for the `stm32f3discovery` board.

Includes a small fix to the `cortex-m4_common` code that is necessary for this PR to work.
Not part of this PR, but shouldn't we outsource this to something like `cortex_m3_common`?