buildroot

system: move the rootfs skeleton choice

In the following commits, we'll be switching more options to be
conditional on the default or custom skeleton.

So, it makes sense that those options come after the choice of a
skeleton.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>