config: bootloader: Add support for MCUboot

Add a new Kconfig option, BOOTLOADER_MCUBOOT, that automatically sets
the required options necessary to make the resulting image bootable by
the MCUboot open source bootloader. This includes the text section
offset and the vector relay table for Cortex-M0, and in the future it
might also add the DTS overlay required to link at slot0 offset in
flash.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>