image_mount_if: make sure the mount point exists

Makefile

@@ -454,6 +454,7 @@ $(IMAGE_DIR)$(GRUB_INSTALL_DIR)/lipinfo.cfg : | $(WORKSPACE)
 	echo "set lip_extra_info=\"$(IMAGE_EXTRA_INFO)\"" >> "$@"
 
 image_mount_if : $(IMAGE_PART_FILE)
+	mkdir -p "$(IMAGE_DIR)"
 	findmnt --target "$(IMAGE_DIR)" || mount "$(IMAGE_PART_FILE)" "$(IMAGE_DIR)"
 
 image_umount :