boards: Add support for the BBC micro:bit nRF51 board

Board uses nRF51822-QFAA, with 16kB of RAM and 256kB of flash.
The UART has no hardware flow control pins.

Change-Id: I16ffeee15a1f5714c695dc8b38e77fb134ea7a0f
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>