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`
Just use a `for (int i; i < size; ++i) {…} return size`. This pattern here hurts readability.