[nrf fromtree] modules: mcuboot: Add Kconfig for firmware updater image

Adds a Kconfig which can be selected when building for firmware
updater mode to select if this is the application or the firmware
updater image that is being built

Signed-off-by: Jamie McCrae <[email protected]>
(cherry picked from commit 890787d)

Author: nordicjm

modules/Kconfig.mcuboot

@@ -256,6 +256,16 @@ config MCUBOOT_BOOTLOADER_NO_DOWNGRADE
 	  MCUBOOT_DOWNGRADE_PREVENTION option enabled.
 endif
 
+config MCUBOOT_APPLICATION_FIRMWARE_UPDATER
+	bool "Application is firmware updater image"
+	depends on MCUBOOT_BOOTLOADER_MODE_FIRMWARE_UPDATER
+	help
+	  Select this if the current image is the firmware updater image. This will use slot 1
+	  information when signing the image.
+
+	  Note that the zephyr chosen node ``zephyr,code-partition`` should be set to
+	  ``slot1_partition`` for this image.
+
 endmenu # On board MCUboot operation mode
 
 endif # BOOTLOADER_MCUBOOT