boot/zephyr: Kconfig changes supporting single image DFU
boot/zephyr: Kconfig changes supporting single image DFU

Modifications to Kconfig that add option for selection of single image
DFU and disable incompatible options.

Signed-off-by: Dominik Ermel <1a1d45a9cc0c98a37f8d0a0d2dbe3cacc0b2344f@nordicsemi.no>

ATmobica/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,utzig/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,utzig/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,tamban01/mcuboot,utzig/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,utzig/mcuboot,utzig/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot