hooks: update motd handling and remove 50-motd-news

The base-files package grew a new /etc/update-motd.d/50-motd-news
script during the lifetime of Core 16. This script wants to write
to the non-writable /var/cache/motd-news location and also needs
curl to work. So it fails and that leads to breaking the
/etc/update-motd.d/00-header script.

This commit disables the script and also adds a check to ensure
that no new files are added to /etc/update-motd.d without us
noticing.

Author: mvo5

live-build/hooks/14-set-motd.chroot

@@ -20,3 +20,18 @@ EOF
 # in particular the 'help' message but leave the 'Welcome'
 # string which is autogenerated based on uname -a and such.
 rm /etc/update-motd.d/10-help-text
+
+# Ensure there is no motd-news. If that file is there the
+# /etc/update-motd.d/50-motd-news script will try to fetch news from
+# motd.ubuntu.com but will fail and cause /etc/update-motd.d/00-header
+# not to run.
+#
+# See also: https://github.com/snapcore/core18/pull/168
+! test -e /etc/default/motd-news
+
+# fail if anything in update-motd.d changed so that we can inspect/update
+if [ "$(find /etc/update-motd.d/ -type f | wc -l)" != 2 ]; then
+    echo "The /etc/update-motd.d/ directory content changed, please inspect:"
+    ls -l /etc/update-motd.d
+    exit 1
+fi