add systemd support

This adds optional systemd support. When enabled, an utility called
psplash-systemd is built. This tool will connect to systemd using the
system bus to obtain progress information and communicate that to
psplash. Once full boot progress is reported by systemd psplash
psplash is closed using the quit command.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>