cmake: Fix shield bug when BOARD_ROOT is a list

Fix bug where SHIELDS would be marked as "not found" in the
NOT_FOUND_SHIELD_LIST when they were present, just not in the first
BOARD root in the BOARD_ROOT list.

Instead of marking shields in the NOT_FOUND_SHIELD_LIST, we (continue)
popping shields from SHIELD and check if there are any shields left in
SHIELD.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>