shields: Modify overlay file order

Shield overlay file has to be taken into account after board overlay
file.
ex: When a new node need to be created for a specific board (in
boards/shields/shield_name/boards/board_name/*.overlay) and used by the
shield (in boards/shields/*.overlay), the order of the overlay files is
important

Signed-off-by: Ismael Fillonneau <ismael.fillonneau@stimio.fr>