Run systemd-tmpfiles as part of the build

Run systemd-tmpfiles as part of the build

Let's make sure we take user provided tmpfiles snippets into account
as well. Since systemd now mounts the initramfs read-only by default,
we need to make sure all tmpfiles snippets that copy to /etc have
already been processed during the image build itself as they won't be
able to run during the initramfs stage.