board: Initial import of stm32f4discovery board

~~This is still WIP, left TODOs are:~~
 - [x] testing of GPIO, UART and timers
 - [x] removing compiler warnings
 - [x] cleaning up documentation (and comment styles)
 - [x] disabling newlib-nano specs by default
 - [ ] handling of FPU (saving and restoring FPU context) -> see issue #1366
 - [x] remodel `cortexm_common` into `cortex-m4_common`
If it is legal is not the question, the question is if this is a good placeholder. I'd say: no. https://github.com/RIOT-OS/RIOT/blob/master/core/include/arch/lpm_arch.h#L46 contains the named return values. If LPM is not implemented yet, then just use `LPM_ON` or `LPM_UNKNOWN`, but not a meaningless 0.