[nrf noup] boot: zephyr: Add NCS boot banner

Adds a boot banner which shows as MCUboot

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
(cherry picked from commit 4b3d6ab)

Author: nordicjm

boot/zephyr/prj.conf

@@ -36,3 +36,6 @@ CONFIG_CBPRINTF_NANO=y
 CONFIG_PICOLIBC=y
 ### Disable malloc arena because we don't need it
 CONFIG_COMMON_LIBC_MALLOC_ARENA_SIZE=0
+
+# NCS boot banner
+CONFIG_NCS_APPLICATION_BOOT_BANNER_STRING="MCUboot"