board: Initial import of stm32f4discovery board (WIP)

This is still WIP, left TODOs are:
 - [ ] testing of GPIO, UART and timers
 - [ ] removing compiler warnings
 - [ ] cleaning up documentation (and comment styles)
 - [x] disabling newlib-nano specs by default
 - [ ] handling of FPU (saving and restoring FPU context)
 - [x] remodel `cortexm_common` into `cortex-m4_common`
Don't you need to call and remember the result of __get_PRIMASK() before calling __enable_irq()?