realtek: image: add zynos-firmware recipe

Add a build recipe to build a ZyNOS firmware image using mkzynfw from
firmware-utils to produce an image that can be flashed with the web
interface of ZyNOS vendor firmware.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/22909
Signed-off-by: Robert Marko <robimarko@gmail.com>

target/linux/realtek/image/Makefile

@@ -17,7 +17,9 @@ DEVICE_VARS += \
         H3C_DEVICE_ID \
         H3C_PRODUCT_ID \
 	LINKSYS_HEADER \
-        ZYXEL_VERS
+	ZYNFW_BOARD \
+	ZYNFW_ALIGN \
+	ZYXEL_VERS
 
 define Build/rt-compress
 	$(STAGING_DIR_HOST)/bin/xz -9 --format=lzma --stdout "$@" > "$@.new"
@@ -168,6 +170,15 @@ define Device/rt-loader-bootbase
 	KERNEL_INITRAMFS := $$(KERNEL/rt-compress) | uImage lzma | rt-loader
 endef
 
+define Build/zynos-firmware
+	$(STAGING_DIR_HOST)/bin/mkzynfw \
+	    -B $(ZYNFW_BOARD) \
+	    -b $(KDIR)/loader-$(DEVICE_NAME).bin \
+	    -r $@:$(ZYNFW_ALIGN) \
+	    -o $@.new
+	mv $@.new $@
+endef
+
 include $(SUBTARGET).mk
 
 $(eval $(call BuildImage))