Disentangle enabling universe in the final image a little from having

PREINSTALLED=true set and enable it for a live-server build.
(LP: #1783129)

debian/changelog

@@ -1,3 +1,11 @@
+livecd-rootfs (2.525.8) UNRELEASED; urgency=medium
+
+  * Disentangle enabling universe in the final image a little from having
+    PREINSTALLED=true set and enable it for a live-server build.
+    (LP: #1783129)
+
+ -- Michael Hudson-Doyle <michael.hudson@ubuntu.com>  Thu, 16 Aug 2018 21:28:00 +1200
+
 livecd-rootfs (2.525.7) bionic; urgency=medium
 
   * Disable journald rate limiting in the live-server live session. 

live-build/auto/build

@@ -237,7 +237,7 @@ EOF
 	rm -f chroot/var/cache/debconf/*-old chroot/var/lib/dpkg/*-old
 	Chroot chroot apt clean
 
-	if [ -f config/oem-config-preinstalled ]; then
+	if [ -f config/universe-enabled ]; then
 
 # This is cargo-culted almost verbatim (with some syntax changes for
 # preinstalled being slightly different in what it doesn't ask) from

live-build/auto/config

@@ -156,6 +156,7 @@ case $IMAGEFORMAT in
 				#  by skipping lb_chroot_live-packages.
 				skip_lb_stage chroot_live-packages
 				INITRAMFS_TYPE=auto
+	                        touch config/universe-enabled
 				;;
 			*)
 				PREINSTALLED=true
@@ -218,9 +219,9 @@ case $IMAGEFORMAT in
 esac
 
 if [ "$PREINSTALLED" = "true" ]; then
-	# This is an oem-config preinstalled image, touch a random file that
-	# we can refer back to during build, cause that's wildly hackish
-	touch config/oem-config-preinstalled
+	# Touch a random file that we can refer back to during build,
+	# cause that's wildly hackish
+	touch config/universe-enabled
 	case $PROJECT in
 		kubuntu*)
 			add_package live oem-config-kde ubiquity-frontend-kde