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

[LEDE Commits]

nbd pushed a commit to source.git, branch master:
https://git.lede-project.org/?p=source.git;a=commitdiff;h=2180b715c1dcef30cb8e734c42b7e24f5f6555e5

commit 2180b715c1dcef30cb8e734c42b7e24f5f6555e5
Author: Felix Fietkau <nbd at nbd.name>
AuthorDate: Thu Aug 4 11:12:27 2016 +0200

    image: fix per-device rootfs build error when not all opkg package files are found
    
    Signed-off-by: Felix Fietkau <nbd at nbd.name>
---
 include/image.mk | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/image.mk b/include/image.mk
index f0b25aa..c85a9d6 100644
--- a/include/image.mk
+++ b/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)))