Add compile_update_sdk flag to BuilderConfig.Build.

There are three steps that must be told to use prebuilts, or to compile
from source.  All three need to be separate.

BUG=chromium:1036550
TEST=generate.sh

Change-Id: If416654672b9c44e21356f33302b1389082c17e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/1986880
Tested-by: LaMont Jones <lamontjones@chromium.org>
Reviewed-by: Sean Abraham <seanabraham@chromium.org>
Reviewed-by: David Burger <dburger@chromium.org>
Commit-Queue: LaMont Jones <lamontjones@chromium.org>
3 files changed