toolchain: prepare_for_build additional_args

Certain toolchain artifacts require additional information to generate
and verify the artifact.

BUG=chromium:1019868
TEST=generate

Change-Id: I1c13dd43f4552b0c68bc127bc34b4d8a83223baa
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2048023
Tested-by: LaMont Jones <lamontjones@chromium.org>
Reviewed-by: David Burger <dburger@chromium.org>
Commit-Queue: LaMont Jones <lamontjones@chromium.org>
7 files changed