toolchain: add a second PrepareForBuild endpoint.

This is being added for testing.  One of the two endpoints will be
deprecated and removed.

BUG=chromium:1019868
TEST=generate.sh

Change-Id: Ie57ab329b9360bc140ec034972aee465a2ac08cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/1974773
Tested-by: LaMont Jones <lamontjones@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
Commit-Queue: LaMont Jones <lamontjones@chromium.org>
3 files changed