board: Initial import of stm32f4discovery board (WIP)

This is still WIP, left TODOs are:
 - [ ] testing of GPIO, UART and timers
 - [ ] 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)
 - [x] remodel `cortexm_common` into `cortex-m4_common`
That I thought so too, but the documentation for `irq_arch_enable()` states:
`@return              the IRQ state after enabling interrupts`
So I kept to this...