Add cos-extensions ebuild for go code

Add the ebuild for building cos-extensions
from go code, but not building it yet.

BUG=b/341721911
TEST=presubmit
RELEASE_NOTE=None

Change-Id: I2af67ccf272a2241b3d9ab74718670091bbd902b
Reviewed-on: https://cos-review.googlesource.com/c/cos/overlays/board-overlays/+/78702
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
2 files changed