systemd: rework network/tty units installation

The installation of the network.service and serial-getty.service unit
files are now performed via the <pkg>_INIT_SYSTEMD mechanism instead of
an installation hook.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>