gmec: Add a target-gmec for GMEC specific packages.

This will allow for easier maintainence of GMEC variations.

BUG=b/176202502
TEST=none
RELEASE_NOTE=none

Change-Id: I9d300c68bb89f4d1cd6f7198280fa89396283372
Reviewed-on: https://cos-review.googlesource.com/c/cos/overlays/board-overlays/+/11891
Reviewed-by: Roy Yang <royyang@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
2 files changed