package/skeleton: select it rather than default to y

package/skeleton: select it rather than default to y

Our current skeleton is tailored to sysv-like init systems; it is not
fit for systemd-based systems. So, in upcoming changes, we'll add
another skeleton for systemd.

This means we can no longer have the current skeleton default to 'y', or
it would be enabled also for systemd, which would be incorrect.

So, we remove the default to 'y' but have it selected by the default
skeleton choice.

However, we do not yet have a way to directly build (really, install)
the custom skeleton, it is built (really, installed) as a dependency of
the default skeleton. So we must also forcibly select the default
skeleton when using a custom one.

Until we have the means to do only one or the other; i.e. when we have a
virtual skeleton.

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>