cmdlib.sh: Switch to using OSTree layers for overlay/

So now that we have OSTree layer capabilities in rpm-ostree
(coreos/rpm-ostree#1830), I think it
makes more sense overall to make the overlay an OSTree layer.

The fact that it *is* an RPM, but differs from all the other RPMs in
various ways will (and already has) come up multiple times when we talk
about signing, changelogs, lockfiles, overrides, etc... (See [1] and [2]
for some already filed issues). Switching to an OSTree layer sidesteps
many of those issues.

Of course, we do lose some things from moving away from RPM:
1. it's harder to determine from where the overlay files come from on a
   host system (see [3] related to this)
2. invisible to `rpm-ostree db diff` (but then again, the diff we
   currently get isn't helpful)
3. not possible to issue an advisory for it to be consumed like the rest
   of the advisories (though with the current approach, making use of
   this without going through Bodhi would've required some non-trivial
   amount of work too)

Overall though, I think those are things we could (and should strive to)
fix in rpm-ostree rather than avoiding layers entirely.

[1] coreos#544
[2] coreos#553 (comment)
[3] coreos/rpm-ostree#1789

src/cmdlib.sh

@@ -254,13 +254,13 @@ EOF
         echo 'ref: "'"${ref}"'"' >> "${override_manifest}"
     fi
     if [ -d "${configdir}/overlay" ]; then
+        echo -n "Committing ${configdir}/overlay... "
+        ostree commit --repo="${tmprepo}" --tree=dir="${configdir}/overlay" -b "${name}-overlay" \
+          --owner-uid 0 --owner-gid 0 --no-xattrs --no-bindings --parent=none
         cat >> "${override_manifest}" <<EOF
-packages:
+ostree-layers:
   - ${name}-overlay
 EOF
-        mkdir -p "${overridesdir}"/rpm
-        mkdir tmp/overlay-build
-        (cd tmp/overlay-build && "${DIR}"/build_rpm_from_dir "${configdir}/overlay" "${name}-overlay" "${workdir}/overrides/rpm")
     fi
     if [[ -n $(ls "${overridesdir}/rpm/"*.rpm 2> /dev/null) ]]; then
         (cd "${overridesdir}"/rpm && createrepo_c .)