build: image: add UBIFS kernel packer

This allows packing the kernel into UBIFS like newer
MikroTik NAND devices require.

Signed-off-by: Robert Marko <robimarko@gmail.com>

include/image-commands.mk

@@ -27,6 +27,15 @@ define Build/append-kernel
 	dd if=$(IMAGE_KERNEL) >> $@
 endef
 
+define Build/package-kernel-ubifs
+	mkdir $@.kernelubifs
+	cp $@ $@.kernelubifs/kernel
+	$(STAGING_DIR_HOST)/bin/mkfs.ubifs \
+		$(KERNEL_UBIFS_OPTS) \
+		-r $@.kernelubifs $@
+	rm -r $@.kernelubifs
+endef
+
 define Build/append-image
 	dd if=$(BIN_DIR)/$(DEVICE_IMG_PREFIX)-$(1) >> $@
 endef