system: move setting getty to the corresponding init systems

system: move setting getty to the corresponding init systems

Currently, setting the getty is done:
  - by the skeleton package when the init system is either busybox or
    sysvinit;
  - by the systemd package when the init system is systemd;
both by registering a target-finalize hook.

This is not very consistent.

Move setting the getty out of the skeleton and into the package that
provides the init system, by registering a per-package target-fialize
hook.

This offloads yet a bit more out of the skeleton, so that it is easier
to properly separate the skeletons for the various init systems.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>