toolchain: Add input_artifacts.

The toolchain builders frequently need to consume artifacts from
previous stages.  Add configuration and Build API support for that.

BUG=chromium:1019868
TEST=generate.sh

Change-Id: I8c7fb78a0a3956eeae630d6b27436eb501acc269
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/1988529
Reviewed-by: Alex Klein <saklein@chromium.org>
Tested-by: LaMont Jones <lamontjones@chromium.org>
Commit-Queue: LaMont Jones <lamontjones@chromium.org>
5 files changed