Wait for udev to process all the events during volatile.img preparation

Otherwise it could happen that the device will still be opened by udev,
so can't be freed by kpartx

linux/aux-tools/prepare-volatile-img.sh

@@ -40,7 +40,7 @@ kpartx -s -a "$FILENAME"
 loopdev=`losetup -j "$FILENAME"|tail -n 1 |cut -d: -f1`
 looppart=`echo $loopdev|sed 's:dev:dev/mapper:'`
 mkswap -f ${looppart}p1 > /dev/null
-sync
+udevadm settle
 kpartx -s -d ${loopdev}
 losetup -d ${loopdev} || :
 chown --reference `dirname "$FILENAME"` "$FILENAME"