Fix list concatenation

Change-Id: I39c2ace7c2daff8c122ba1e256080771e46a0907
diff --git a/vars/functions.groovy b/vars/functions.groovy
index 6e7180f..ee41cf1 100644
--- a/vars/functions.groovy
+++ b/vars/functions.groovy
@@ -67,7 +67,7 @@
         }
         
         def filenames = (packages + sources).each{entry -> entry.path}
-        packageFilenames = packageFilenames + filenames
+        packageFilenames.addAll(filenames)
     }