Skip userdata.img only if partition size is not give for extfs.

We still build yaffs2 images which don't need partition size.

Change-Id: I9c7f45069f25868666a257cee044faecf1a8781a
diff --git a/tools/releasetools/img_from_target_files b/tools/releasetools/img_from_target_files
index 3a52342..007a3f7 100755
--- a/tools/releasetools/img_from_target_files
+++ b/tools/releasetools/img_from_target_files
@@ -57,8 +57,9 @@
 
   image_props = build_image.ImagePropFromGlobalDict(OPTIONS.info_dict,
                                                     "data")
-  # If no userdata_size is provided, skip userdata.img.
-  if not image_props.get("partition_size"):
+  # If no userdata_size is provided for extfs, skip userdata.img.
+  if (image_props.get("fs_type", "").startswith("ext") and
+      not image_props.get("partition_size")):
     return
 
   print "creating userdata.img..."