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.serial_recovery b/boot/zephyr/Kconfig.serial_recovery
index c73badd..74bced7 100644
--- a/boot/zephyr/Kconfig.serial_recovery
+++ b/boot/zephyr/Kconfig.serial_recovery
@@ -13,6 +13,7 @@
 	select BASE64
 	select CRC
 	select ZCBOR
+	depends on !BOOT_FIRMWARE_LOADER
 	help
 	  If y, enables a serial-port based update mode. This allows
 	  MCUboot itself to load update images into flash over a UART.