image: fix per-device rootfs build error when not all opkg package files are found

Signed-off-by: Felix Fietkau <nbd@nbd.name>

include/image.mk

@@ -294,7 +294,7 @@ target-dir-%: FORCE
 	$(CP) $(TARGET_DIR) $(mkfs_cur_target_dir)
 	mv $(mkfs_cur_target_dir)/etc/opkg $(mkfs_cur_target_dir).opkg
 	echo 'src default file://$(PACKAGE_DIR_ALL)' > $(mkfs_cur_target_dir).conf
-	$(if $(mkfs_packages_add), \
+	$(if $(call opkg_package_files,$(mkfs_packages_add)), \
 		$(opkg_target) update && \
 		$(opkg_target) install \
 			$(call opkg_package_files,$(mkfs_packages_add)))