When the Kconfig BOOTLOADER_MCUBOOT is selected, an overlay to place the
image at the slot0 location is required. In order to avoid having to do
this manually for all samples when targetting MCUboot, include the logic
inside the dts.cmake script to prepend a new common.dts file that then
conditionally includes mcuboot.overlay.
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>