Disable merged /usr in buildd images

Otherwise we might end up building packages with hardcoded paths that
assume a merged /usr.

debian/changelog

@@ -1,3 +1,9 @@
+livecd-rootfs (2.560) UNRELEASED; urgency=medium
+
+  * Disable merged /usr in buildd images.
+
+ -- Colin Watson <cjwatson@ubuntu.com>  Fri, 01 Feb 2019 18:01:56 +0000
+
 livecd-rootfs (2.559) disco; urgency=medium
 
   [ Jean-Baptiste Lallement ]

live-build/auto/config

@@ -1220,6 +1220,11 @@ esac
 case $SUBPROJECT in
 	buildd)
 		cp -af /usr/share/livecd-rootfs/live-build/buildd/* config/
+
+		# Disable merged /usr to avoid building packages with
+		# hardcoded paths that assume it.
+		echo 'DEBOOTSTRAP_OPTIONS="$DEBOOTSTRAP_OPTIONS --no-merged-usr"' \
+			>> config/common
 		;;
 esac