prototype: Add build target configs

Adding build target configs to outer config proto to support generating
via starlark config.

BUG=None
TEST=None

Change-Id: Ib5b5d315d2784e97f41c1f9d0c38334bf65e240c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/1985809
Reviewed-by: C Shapiro <shapiroc@chromium.org>
Commit-Queue: C Shapiro <shapiroc@chromium.org>
Tested-by: C Shapiro <shapiroc@chromium.org>
Auto-Submit: C Shapiro <shapiroc@chromium.org>
5 files changed