Fixing condition for setting rootfs_dest_backing.

The fix for the opt_link check in d5ed942 broke the code as it inverted the
meaning. Previously it ran the code iff opt_link was empty, as [ -n ] is
true. In the case opt_link was not empty, the code evaluated to [ -n foo],
which is invalid syntax and thus false. This, however, was almost the expected
behaviour, as rootfs_dest_backing is needed for opt_union and (in read-only
mode) for opt_link.

scripts/boot/9990-misc-helpers.sh

@@ -1482,7 +1482,7 @@ activate_custom_mounts ()
 		# ignore the loop below and set rootfs_dest_backing=$dest
 		local rootfs_dest_backing
 		rootfs_dest_backing=""
-		if [ -n "${opt_link}" ]
+		if [ -n "${opt_link}" ] || [ -n "${opt_union}" ]
 		then
 			for d in /live/rootfs/*
 			do