system: separate sysv and systemd parts of the skeleton

system: separate sysv and systemd parts of the skeleton

For systemd, we create a simple /etc/fstab with only an entry for /, as
systemd otherwise automatically mounts what it needs where it needs it.

systemd does not like that the content of /var be symlinks to /tmp,
especially journald that starts before /tmp is mounted, and thus the
journal files are hidden from view, which causes quite a bit of fuss...

Instead, move the current /var to a sysv-only skeleton.

systemd at install time will create the /var content it needs, so we
just create an empty /var for systemd.

systemd would create /home and /srv at runtime if they are missing, but
it is better to create them right now, to simplify supporting systemd on
a RO filesystem in the (near) future.

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