feat(metadata): Add support for updating ebuild metadata cache

New script update_metadata to do a global update of both repos.
Add support to update_ebuilds to do incremental updates when just
pulling new ebuilds into portage-stable.

First refresh of the cache is in https://github.com/coreos/coreos-overlay/pull/677 and https://github.com/coreos/portage-stable/pull/115

This speeds up the first emerge command in a fresh environment quite a bit.