yum-minimal: workaround missing $releasedir variable

As described inline, work around missing $releasedir variables seen in
production builds.

Change-Id: Ia114f41d7a2fa86957235996f10a74adeecf767c

diskimage_builder/elements/yum-minimal/root.d/08-yum-chroot

@@ -324,6 +324,25 @@ function _run_chroot {
     sudo -E chroot $TARGET_ROOT env -u TMPDIR sh -c "$cmd"
 }
 
+# The rpmdb has been created by the host RPM.  CentOS 7 only
+# understands bdb-based db's, while the host is (likey as not) a more
+# modern rpm that has created a sqlite db.  These don't share files in
+# common, so to the in-chroot rpm the db just looks empty.  This is a
+# super-weird state that the system is in, because everything is
+# installed and working, but packages don't look like they are.  One
+# consequence of this is that yum's querying to setup the $releasever
+# variable fails and it remains unset.  Because the default .repo
+# files use this we get invalid repo paths for any yum commands.  The
+# easiest way around this seems to be to manually set --releasever=7;
+# this way yum can rebuild itself and recreate the rpmdb as it likes.
+# This is a mess that can hopefully go away when we don't care about
+# CentOS 7.  For this reason, we only do this for CentOS 7, to avoid
+# hiding any problems on other distros.  We only need to do this
+# for these initial steps, after that the db is correct.
+if [[ ${DISTRO_NAME} = centos && ${DIB_RELEASE%-stream} -le 7 ]]; then
+    YUM="${YUM} --releasever=${DIB_RELEASE}"
+fi
+
 # we just installed yum/dnf with "outside" tools (yum/rpm) which
 # might have created /var/lib/[yum|rpm] (etc) that are slighlty
 # incompatible.  Refresh everything with the in-chroot tools