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