boards/arm: Add mcuboot's led alias to nRF boards

Added alias (mcuboot-led0) for the LED gpio pin which might be
used by MCUboot.
So far this pin assignments for MCUboot was configured by
bootloader-led0 alias.
New alias will replace the bootloader-led0.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>