zephyr: Add firmware loader MCUboot operation style

Adds a new operation style in which the secondary slot has an
image which is used to update the primary image only.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
diff --git a/boot/zephyr/Kconfig b/boot/zephyr/Kconfig
index 183bb50..a67126a 100644
--- a/boot/zephyr/Kconfig
+++ b/boot/zephyr/Kconfig
@@ -256,6 +256,18 @@
 	  The address that the image is copied to is specified using the load-addr
 	  argument to the imgtool.py script which writes it to the image header.
 
+config BOOT_FIRMWARE_LOADER
+	bool "Firmware loader"
+	help
+	  If y, mcuboot will have a single application slot, and the secondary
+	  slot will be for a non-upgradeable firmware loaded image (e.g. for
+	  loading firmware via Bluetooth). The main application will boot by
+	  default unless there is an error with it or the boot mode has been
+	  forced to the firmware loader.
+
+	  Note: The firmware loader image must be signed with the same signing
+	  key as the primary image.
+
 endchoice
 
 # Workaround for not being able to have commas in macro arguments
@@ -582,6 +594,8 @@
 
 rsource "Kconfig.serial_recovery"
 
+rsource "Kconfig.firmware_loader"
+
 config BOOT_INTR_VEC_RELOC
 	bool "Relocate the interrupt vector to the application"
 	default n