toolchain: updates to PrepareForBuild

Also add some clarifying comments to Build.compile_toolchain and
compile_source.

BUG=chromium:1019868
TEST=generate.sh

Change-Id: Ic798969cfd2c41a9d48cef0635ad3b77c87c50c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/1955691
Tested-by: LaMont Jones <lamontjones@chromium.org>
Reviewed-by: David Burger <dburger@chromium.org>
Reviewed-by: Sean Abraham <seanabraham@chromium.org>
Commit-Queue: LaMont Jones <lamontjones@chromium.org>
5 files changed