Fix buildd images for series earlier than bionic

Don't install gpg or gpg-agent when building buildd images for series
earlier than bionic.  (In earlier series, those packages didn't exist
and gnupg was build-essential.)

debian/changelog

@@ -1,3 +1,11 @@
+livecd-rootfs (2.557) UNRELEASED; urgency=medium
+
+  * Don't install gpg or gpg-agent when building buildd images for series
+    earlier than bionic.  (In earlier series, those packages didn't exist
+    and gnupg was build-essential.)
+
+ -- Colin Watson <cjwatson@ubuntu.com>  Wed, 16 Jan 2019 12:36:48 +0000
+
 livecd-rootfs (2.556) disco; urgency=medium
 
   * Add wpasupplicant to the additional packages installed for the raspi2 and

live-build/auto/config

@@ -686,8 +686,15 @@ case $SUBPROJECT in
 		add_package install policyrcd-script-zg2
 		add_package install pkgbinarymangler
 		add_package install ca-certificates
-		add_package install gpg
-		add_package install gpg-agent
+		case $SUITE in
+			precise|trusty|xenial)
+				;;
+			*)
+				# only needed in >= bionic
+				add_package install gpg
+				add_package install gpg-agent
+				;;
+		esac
 		case $SUITE in
 			precise|trusty|xenial)
 				# no longer needed in >= artful