The ifupdown scripts can be used independently of the init system, be it
sysv, busybox or systemd; they could even be used when there is no init
system (i.e. the user is providing his own).

Currently, those ifupdown scripts are bundled in the skeleton.

But we soon will have a skeleton specific to systemd, so we would be
missing those scripts (when systemd-networkd is not enabled).

So, move those scripts to their own package.

To keep the current behaviour (before it is changed in future commits),
we make that package default to y, but depend on the default skeleton.

Instead of being a target-finalize hook, the scripts are installed as
any other package are, with a package install-target command.

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