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/board: improved UART impl. for nrf51822

- effects boards: pca10000 and pca10005
[SQUASH ME] cpu/nrf: fixed stupid error