dfu/boot: add CONFIG_UPDATEABLE_IMAGE_NUMBER option
dfu/boot: add CONFIG_UPDATEABLE_IMAGE_NUMBER option

This option is deciding whether bootutil_public library supports
multi-image procedures. MCUboot have its own definition which is
unavailable zephyr application. Introduced option declares similar
option for the application.

Signed-off-by: Andrzej Puzdrowski <0222c24e1773dd88269ed0e559909ec9bea3604a@nordicsemi.no>

zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr