systemd: Enable specifying extra config files for a unit

This will allow overriding package-provided units, or overriding only a
specific instance of a unit template.

Signed-off-by: Shea Levy <shea@shealevy.com>
kmscon: Don't re-run systemd-vconsole-setup after boot, and let tty1 wait for vconsole setup

Signed-off-by: Shea Levy <shea@shealevy.com>