Import patches-unapplied version 2.493 to ubuntu/bionic-proposed

Imported using git-ubuntu import.

Changelog parent: 3d8c4a0b35

New changelog entries:
  * Set a default archive mirror - needed for germinate.

debian/changelog

@@ -1,3 +1,9 @@
+livecd-rootfs (2.493) bionic; urgency=medium
+
+  * Set a default archive mirror - needed for germinate.
+
+ -- Iain Lane <iain.lane@canonical.com>  Tue, 30 Jan 2018 15:08:23 +0000
+
 livecd-rootfs (2.492) bionic; urgency=medium
 
   * Oops. I shouldn't reference my private stuff.

live-build/auto/config

@@ -30,6 +30,7 @@ if [ -z "$MIRROR" ]; then
 		*.buildd)		MIRROR=http://ftpmaster.internal/ubuntu/
 					SEEDMIRROR=http://archive-team.internal/seeds/
 					;;
+		*)			MIRROR=http://archive.ubuntu.com/ubuntu/ ;;
 	esac
 fi