packages: Add after-install target unit

Units that get installed via `rpm-ostree` on first boot cannot be
enabled by ignition, because they do not exist when it runs `systemctl
preset`.  Thus, anything we want to start after its been installed needs
to be explicitly started.  To allow this in an extensible fashion, I've
added an `after-install.target` unit and modified the
`install-packages.sh` script to activate this unit once the installation
is complete.  The script also re-runs `systemctl preset`, so services
will start automatically on subsequent boots.

after-install.target

@@ -0,0 +1,2 @@
+[Unit]
+Description=Start services after package install

install-packages.sh

@@ -6,3 +6,6 @@ if [ ! -d /etc/ignition/packages.d ]; then
 fi
 
 cat /etc/ignition/packages.d/* | xargs rpm-ostree install --apply-live -y
+
+systemctl preset-all --preset-mode=enable-only
+systemctl start after-install.target

packages.yaml

@@ -14,6 +14,10 @@ storage:
     mode: 0644
     contents:
       local: install-packages.service
+  - path: /etc/systemd/system/after-install.target
+    mode: 0644
+    contents:
+      local: after-install.target
 
 systemd:
   units: