image: add recipe for legacy multiple images

Asus RT-AX89X has modded U-Boot that boots FIT images fine when manually
using bootm command, however once the image is flashed and device boots via
the default bootcmd it only supports booting via legacy images.

More precisely, it requires the "multi" image format with:
1. kernel
2. ramdisk
3. DTB

So, lets add a recipe based on the existing uImage one.

Link: https://github.com/openwrt/openwrt/pull/15840
Signed-off-by: Robert Marko <robimarko@gmail.com>

include/image-commands.mk

@@ -701,6 +701,23 @@ define Build/uImage
 	mv $@.new $@
 endef
 
+define Build/multiImage
+	$(if $(UIMAGE_TIME),SOURCE_DATE_EPOCH="$(UIMAGE_TIME)") \
+	mkimage \
+		-A $(LINUX_KARCH) \
+		-O linux \
+		-T multi \
+		-C $(word 1,$(1)) \
+		-a $(KERNEL_LOADADDR) \
+		-e $(if $(KERNEL_ENTRY),$(KERNEL_ENTRY),$(KERNEL_LOADADDR)) \
+		-n '$(if $(UIMAGE_NAME),$(UIMAGE_NAME),$(call toupper,$(LINUX_KARCH)) $(VERSION_DIST) Linux-$(LINUX_VERSION))' \
+		$(if $(UIMAGE_MAGIC),-M $(UIMAGE_MAGIC)) \
+		-d $@:$(word 2,$(1)):$(word 3,$(1)) \
+		$(wordlist 4,$(words $(1)),$(1)) \
+		$@.new
+	mv $@.new $@
+endef
+
 define Build/xor-image
 	$(STAGING_DIR_HOST)/bin/xorimage -i $@ -o $@.xor $(1)
 	mv $@.xor $@