make_image: unbreak parallel calls to make_image

Argh, broke this earlier when tweaking for image count

tools/make_image

@@ -19,12 +19,12 @@ NUM_ARCHES=`echo $ARCHES | wc -w`
 
 if [ "$CD"x = ""x ] ; then
     echo "Generating the $ARCHES iso/jigdo images ..."
+    FILES=$DIR/*.volid
 else
     echo "Generating $ARCHES iso/jigdo image number $CD ..."
+    FILES=$DIR/$CD.volid
 fi
 
-FILES=$DIR/*.volid
-
 # Grab the xorriso version and turn it into a number we can use
 xorriso_version() {
     $MKISOFS --version 2>&1 | awk '