Fix live-persist --media option handling.

Until now, the value passed to this option was, well, ignored.

config/chroot_local-includes/usr/local/sbin/live-persist

@@ -311,7 +311,7 @@ main ()
 				set -x
 				;;
 			--media=*)
-				export PERSISTENCE_MEDIA="${ARGUMENT#*=}"
+				export PERSISTENCE_MEDIA="${1#*=}"
 				;;
 			--read-only)
 				export PERSISTENCE_READONLY="true"