Allow Portage to generate a new metadata file called ROOT

The file ROOT stores the install root path of the package.

BUG=b:226160872
TEST=build_packages+build_image for chell

Change-Id: Id6e87cc56e357dc66a62c4c1d6727c704545d3c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/portage_tool/+/4425874
Tested-by: Gilberto Contreras <gcontreras@google.com>
Commit-Queue: Gilberto Contreras <gcontreras@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
2 files changed