include a sanity check that no layered image has more than one "live" layer

This would cause undefined behavior with the LAYERFS_LIVE handling in the
hook.

debian/changelog

@@ -4,6 +4,9 @@ livecd-rootfs (24.04.15) UNRELEASED; urgency=medium
     layer ending in .live so this can be reused across flavors
   * live-build/ubuntu/hooks/020-ubuntu-live.chroot_early: check for
     glib-compile-schemas presence before calling
+  * live-build/auto/config: include a sanity check that no layered image has
+    more than one "live" layer that would cause undefined behavior with the
+    above hook.
 
  -- Steve Langasek <steve.langasek@ubuntu.com>  Wed, 20 Dec 2023 19:45:34 -0800
 

live-build/auto/config

@@ -107,6 +107,18 @@ _register_pass () {
 	# $1 Name of the pass
 	[ "$PASSES_TO_LAYERS" != "true" ] && return
 
+	# live-build/ubuntu/hooks/020-ubuntu-live.chroot_early assumes the
+	# layer ending in '.live' is THE live layer to use, so ensure that
+	# we only define a single layer ending in '.live'.  It principle it
+	# is not invalid to have multiple layers with .live in the name but
+	# we should not let this happen accidentally.
+	case "$PASSES $1" in
+		*.live *.live)
+			EXPLODE
+			;;
+		*)
+			;;
+	esac
 	PASSES="$PASSES $1"
 }