board/cpu: Import of STM32F3discovery (Cortex-M4) port

Finally here it is: a clean and working (as far as I tested it) port for the STM32F3discovery board. This might not be the end of the line as it can not be merged as is. But hopefully the dependencies can be merged in the near future so this PR can be merged as well.

It includes 
- pkg: stm32f3_stdperiph_lib
- cpu: stm32f303vc
- cpu: cortexm_common (this should be re-usable for all cortex-m's and contains all the 'hard' parts of the port -> thanx to @StefanPfeiffer)

The port is not working stand-alone, it is based on a bunch of open PRs, namely:
- #507
- ~~#613~~
- ~~#614~~
- #920
- #922
- #931