kconfig: hide option for mcuboot support if image is mcuboot
kconfig: hide option for mcuboot support if image is mcuboot

Don't show the option for MCUBoot bootloader support
when the image being built is MCUBoot itself.

Signed-off-by: Håkon Øye Amundsen <4238ca2ac515887d8a25d5e30932b12519303bf7@nordicsemi.no>

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