Add support for passing snap channel to ubuntu-core image builds.

debian/changelog

@@ -1,10 +1,14 @@
 livecd-rootfs (2.489) UNRELEASED; urgency=medium
 
+  [ Balint Reczey ]
   * Revert to building minimized and not minimimized images with the same
     ~2.2 GiB size
   * Stop suggesting partimage which is not used anymore
   * Drop Nexus 7 support
 
+  [ Steve Langasek ]
+  * Add support for passing snap channel to ubuntu-core image builds.
+
  -- Balint Reczey <rbalint@ubuntu.com>  Sat, 06 Jan 2018 23:49:15 +0100
 
 livecd-rootfs (2.488) bionic; urgency=medium

live-build/auto/build

@@ -21,8 +21,9 @@ PREFIX="livecd.$PROJECT${SUBARCH:+-$SUBARCH}"
 if [ "${IMAGEFORMAT:-}" = "ubuntu-image" ]; then
 	# Use ubuntu-image instead of live-build
 
+	CHANNEL="${CHANNEL:-edge}"
 	env SNAPPY_STORE_NO_CDN=1 \
-	ubuntu-image -c edge $UBUNTU_IMAGE_ARGS \
+	ubuntu-image -c "$CHANNEL" $UBUNTU_IMAGE_ARGS \
 		-o "$PREFIX".img "$PREFIX".model-assertion
 	xz -0 -T4 "$PREFIX".img
 	mv seed.manifest "$PREFIX".manifest