boards/arm: Add mcuboot's button alias to Nordic's boards

Added alias for the button's gpio pin which might be used by MCUboot.
So far pin assignments for MCUboot was configured by the Kconfig. New
alias will help to switch configuration to more native DTS based
description.

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