cpu/nrf51822: put stack sized to safer values

the transceiver module is likely to crash on the old values...
cpu/nrf51822: added support for NRF51 radio
baord/airfy-beacon: enabled NRF51 radio
board/pca10000: enabled NRF51 radio
board/pca10005: enabled NRF51 radio
board/yunjia-nrf51822: enabled NRF51 radio
sys: added NRF51 radio to the transceiver