system: move tz setup outside of default skeleton clause

Allow time zone setup and installation for configurations
using custom skeletons as well as default skeletons.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>