Use a custom MCUBoot key when building firmware images

[coderbyheart]

CMake Warning at /workdir/ncs/nrf/cmake/mcuboot.cmake:115 (message):
  

        ---------------------------------------------------------
        --- WARNING: Using default MCUBoot key, it should not ---
        --- be used for production.                           ---
        ---------------------------------------------------------

See https://developer.nordicsemi.com/nRF_Connect_SDK/doc/1.3.0/mcuboot/encrypted_images.html
and https://devzone.nordicsemi.com/f/nordic-q-a/88416/mcuboot-sign-the-firmware