build system: really disable building CrEC when not needed

build system: really disable building CrEC when not needed

Enable users to set the EC_EXTERNAL_FIRMWARE config flag, and actively
ignore anything related to EC firmware board names if enabled.

BUG=none
BRANCH=none
CQ-DEPEND=CL:344540
TEST=emerge-samus coreboot works

Change-Id: I02aa1e4bc0c98300105b83a12979e9368a40cbcf
Signed-off-by: Martin Roth <martinroth@chromium.org>
Original-Commit-Id: 4f0b6fd10aa89fbb38bdebf14b8a82d52e9ee233
Original-Change-Id: I39c3038d059ec3d7710b864061fcf83b8d6d4d13
Original-Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Original-Reviewed-on: https://chromium-review.googlesource.com/345584
Original-Reviewed-by: Aaron Durbin <adurbin@google.com>
Original-Commit-Queue: Martin Roth <martinroth@chromium.org>
Original-Trybot-Ready: Martin Roth <martinroth@chromium.org>
Original-Tested-by: Martin Roth <martinroth@chromium.org>
Reviewed-on: https://review.coreboot.org/15938
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>