build: image: explicitly pass --rootfs to append-ubi

Rootfs is now optional in ubinize-image.sh and
requires --rootfs flag instead of just passing the
rootfs image as the argument before ubinize opts.

So, simply add --rootfs flag before the $(IMAGE_ROOTFS).

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

include/image-commands.mk

@@ -112,7 +112,7 @@ define Build/append-ubi
 		$(if $(UBOOTENV_IN_UBI),--uboot-env) \
 		$(if $(KERNEL_IN_UBI),--kernel $(IMAGE_KERNEL)) \
 		$(foreach part,$(UBINIZE_PARTS),--part $(part)) \
-		$(IMAGE_ROOTFS) \
+		--rootfs $(IMAGE_ROOTFS) \
 		$@.tmp \
 		-p $(BLOCKSIZE:%k=%KiB) -m $(PAGESIZE) \
 		$(if $(SUBPAGESIZE),-s $(SUBPAGESIZE)) \