Create the source dir for custom mounts using the 'linkfiles' option.

We can't bootstrap it, but creating it so the user doesn't have to
(and maybe can't due to permissions) is a still convenient.

scripts/live-helpers

@@ -1398,12 +1398,13 @@ activate_custom_mounts ()
 		# dealing with /etc or other system dir.
 		if [ ! -d "${source}" ]
 		then
-			if [ -n "${PERSISTENT_READONLY}" ] || [ -n "${opt_linkfiles}" ]
+			if [ -n "${PERSISTENT_READONLY}" ]
 			then
 				continue
-			elif [ -n "${opt_union}" ]
+			elif [ -n "${opt_union}" ] || [ -n "${opt_linkfiles}" ]
 			then
-				# union's don't need to be bootstrapped
+				# unions and don't need to be bootstrapped
+				# linkfiles dirs can't be bootstrapped in a sensible way
 				mkdir "${source}"
 			else
 				# ensure that $dest is not copied *into* $source