board/cpu: adjusted uart driver implementations

for
- sam3x8e
- stm32f0
- stm32f4
- sam3x8e
- nrf51822
tests/periph_uart: added two UART driver tests

- test for UART driver in blocking mode
- test for UART driver in interrupt driven mode
Merge pull request #1481 from haukepetersen/fix_drivers_uartif

drivers: improved UART driver interface
cpu - sam3x: finished low-level UART driver
cpu/sam3x8e: general fixes

- fixed PendSV priority
- fixed thread_yield in timer
- put in led blinking on hard_fault
[SQUASH ME] cpu/board: more fixed to uart