core/pkg-generic: add variable to skip skeleton dependency

core/pkg-generic: add variable to skip skeleton dependency

We currently skip the skeleton dependency by checking if the current
package is the skeleton.

We are going to introduce more skeleton-related packages, so we
need a way to exclude the skeleton dependency for those, or we'd
get a circular dependency, for the same reason we need to skip
the toolchain dependency.

Instead of checking for all the skeleton-providing packages in the core
infra, add a new package options so that packages can express they do
not need the dependency on the skeleton, like we have an option to avoid
the depednency on the toolchain. The only packages that will use that
option are probably the skeletons, so we need not document this
variable, like we did not document the option to exclude the dependency
on the toolchain.

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