mediatek: filogic: add ASUS RT-AX52 factory-initramfs image generation

This adds the required image receipt to generate a vendor ui compatible
initramfs-factory image, that can be used to flash the final sysupgrade
image.

Signed-off-by: Christoph Krapp <achterin@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/20409
Signed-off-by: Robert Marko <robimarko@gmail.com>

target/linux/mediatek/image/Makefile

@@ -33,6 +33,11 @@ define Device/Default
 	pad-rootfs | append-metadata
 endef
 
+define Build/asus-trx
+	$(STAGING_DIR_HOST)/bin/asusuimage $(wordlist 1,$(words $(1)),$(1)) -i $@ -o $@.new
+	mv $@.new $@
+endef
+
 include $(SUBTARGET).mk
 
 define Image/Build

target/linux/mediatek/image/filogic.mk

@@ -270,6 +270,9 @@ define Device/asus_rt-ax52
   KERNEL_INITRAMFS := kernel-bin | lzma | \
 	fit lzma $$(KDIR)/image-$$(firstword $$(DEVICE_DTS)).dtb with-initrd | pad-to 64k
   IMAGE/sysupgrade.bin := sysupgrade-tar | append-metadata
+  ARTIFACTS := initramfs.trx
+  ARTIFACT/initramfs.trx := append-image-stage initramfs-kernel.bin | \
+	uImage none | asus-trx -v 3 -n $$(DEVICE_MODEL)
 endef
 TARGET_DEVICES += asus_rt-ax52