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.
Dustin