cmake: shields: Simplify shield processing, print shields from modules

Fix issue #67244 in which boards from external modules where not
printed in the help message (displayed for `shields` target or
when trying to build with unknown shield). The SHIELD_LIST variable
was reset for each BOARD_ROOT discovered.

Simplify discovery logic. Discover all shields and set relevant
variables (SHIELD_LIST and SHIELD_DIR_${name}) in one step and
process all the expected boards in the next step.

Signed-off-by: Marek Metelski <marek@metelski.dev>