psplash: install systemd services

This patch provides service files for using psplash on embedded devices
running systemd:

- psplash-start.service: start psplash.
- psplash-quit.service: kill psplash when reaching multi-user.target

The following kernel command line options should also be set:

  systemd.show_status=0 quiet splash

The option "systemd.show_status=0" is required, because, unlike Plymouth,
psplash does not have real systemd integration, i.e. it will not perform:

  kill(1, SIGRTMIN + 21);

Note that no progress messages will be printed on the splash screen.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>