reproducible OpenWrt: try harder to properly cleanup afterwards

Signed-off-by: Holger Levsen <holger@layer-acht.org>

bin/reproducible_openwrt.sh

@@ -53,7 +53,7 @@ node_cleanup_tmpdirs() {
 	echo "$(date -u) - deleting \$TMPBUILDDIR $TMPBUILDDIR"
 
 	if [ "$RUN" = "b2" ]; then
-		umount "$TMPBUILDDIR/build"
+		umount -l "$TMPBUILDDIR/build"
 	fi
 
 	rm -rf "$TMPBUILDDIR"