Fix broken DNS in buildd image builds.

Instead of injecting an empty resolv.conf with an includes.chroot, we'll
inject it in late with a hook. The empty resolv.conf breaks DNS early in
the build, and causes some binary hooks to fail.

debian/changelog

@@ -1,3 +1,9 @@
+livecd-rootfs (2.632) UNRELEASED; urgency=medium
+
+  * Fix broken DNS in buildd image builds.
+
+ -- David Krauser <david.krauser@canonical.com>  Tue, 10 Dec 2019 15:28:12 -0400
+
 livecd-rootfs (2.631) focal; urgency=medium
 
   * Add support for uc20 model assertions.

live-build/buildd/hooks/49-empty-resolv-conf.binary

@@ -0,0 +1,4 @@
+#!/bin/sh
+set -e
+
+chroot chroot truncate -s 0 /etc/resolv.conf

live-build/buildd/hooks/52-linux-virtual-image.binary

@@ -27,10 +27,6 @@ cleanup_linux_virtual() {
 }
 trap cleanup_linux_virtual EXIT
 
-# The base buildd chroot includes an empty resolv.conf
-# We don't want to include that in the bootable images
-rm $mount_d/etc/resolv.conf
-
 # Install dependencies
 env DEBIAN_FRONTEND=noninteractive chroot "$mount_d" apt-get \
     update --assume-yes