Move growfs and autoupdates into tier-1/usr overlay, use preset

Instead of having postprocessing scripts for these, add
them to the overlay. Also instead of hand rolling static systemd
enablement, add and use a preset file for them instead.

(This also makes it easier for downstream container images
 to rerun the presets if they wanted to)

Signed-off-by: Colin Walters <walters@verbum.org>

tier-1/usr/lib/systemd/system-preset/05-bootc.preset

@@ -0,0 +1,7 @@
+# Our fallback
+enable bootc-generic-growpart.service
+
+# We enable this by default just so we can say we have automatic
+# updates on by default, like CoreOS. It's very much intended
+# to be tweaked or replaced outside of trivial scenarios though.
+enable bootc-fetch-apply-updates.timer