Revert "shields: Modify overlay file order"

Fixes: #29074

This reverts commit fc8f639b9a6df12a024b0ee5b452db06d7b72609.

The suggestion provided in #27901 is impacting processing order of
overlay files in a non-logical way, see #29074 discussion for details.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>