Make sure buildd images have a /usr/sbin/policy-rc.d symlink

Installing policyrcd-script-zg2 doesn't quite do this because of the way
that live-build installs its own temporary version of policy-rc.d.  The
only remotely sensible way I can see to deal with this is to create the
symlink manually.

debian/changelog

@@ -1,3 +1,9 @@
+livecd-rootfs (2.408.44) UNRELEASED; urgency=medium
+
+  * Make sure buildd images have a /usr/sbin/policy-rc.d symlink.
+
+ -- Colin Watson <cjwatson@ubuntu.com>  Mon, 18 Feb 2019 14:55:41 +0000
+
 livecd-rootfs (2.408.43) xenial; urgency=medium
 
   * Add a buildd subproject (LP: #1815251).

live-build/buildd/hooks/01-policy-rc-d.binary

@@ -0,0 +1,13 @@
+#! /bin/sh
+set -e
+
+# Put the /usr/sbin/policy-rc.d alternatives symlink in place.  Ordinarily
+# update-alternatives ought to create this when policyrcd-script-zg2 is
+# installed, but that doesn't work because live-build has already installed
+# a dummy one at that point.  The simplest approach is to repair the
+# situation by putting it in place here.
+if [ -L binary/etc/alternatives/policy-rc.d ] && \
+   [ ! -e binary/usr/sbin/policy-rc.d ] && \
+   [ ! -L binary/usr/sbin/policy-rc.d ]; then
+	ln -s /etc/alternatives/policy-rc.d binary/usr/sbin/policy-rc.d
+fi