Allow specifying ROOTFS_FORMAT for rootfs-mount target

Author: gsingh93

Makefile

@@ -293,7 +293,7 @@ rootfs: ext4 cpio
 .PHONY: rootfs-mount
 rootfs-mount:
 	sudo modprobe nbd
-	sudo qemu-nbd -c /dev/nbd0 $(ROOTFS)
+	sudo qemu-nbd -c /dev/nbd0 -f $(ROOTFS_FORMAT) $(ROOTFS)
 	mkdir -p /tmp/rootfs
 	sudo mount /dev/nbd0 /tmp/rootfs