All new Chrome OS build targets should leverage the chromeos-config ("unified build") setup by default. Currently when making a new build target, it's easy to forget to add "USE unibuild", and thus lots of new development can be erroneously done against the old standard.

We should make unified builds the default, with explicit opt-outs for legacy boards.

vapier@ suggested an approach whereby we enable the unibuild flag in chromiumos-overlay/profiles/targets/chromeos/make.defaults and disable it in the individual legacy overlays' make.defaults. The result will be that new build targets will get unibuild by default, even if they don't explicitly add it in their make.defaults.