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.
Some machines may need to install multiple packages for separate use
cases. Requiring each use case to define a systemd unit that runs
`rpm-ostree install` directly would be cumbersome and also quite slow,
as each one would have to run in turn. Instead, now there is a single
*install-packages.service* which installs all of the packages listed in
files in `/etc/ignition/packages.d`. On first boot, all files in that
directory are read and all the packages they list will be installed in a
single `rpm-ostree install` invocation.
Dustin