Revert "Teach Portage to grab portage config from stateful partition."
Revert "Teach Portage to grab portage config from stateful partition."

This reverts commit 1f09a107cea3d5aacf51572cde12c56f1251141e.
Description below.

Teach Portage to grab portage config from stateful partition.

With PORTAGE_CONFIGROOT=/usr/local, Portage looks in /usr/local/etc for config
instead of /etc. This allows us to configure necessary configstuffs in
the stateful partition and avoids the need to copy it over to the rootfs
manually.

I've also updated ROOT to only be a default so that if the user can set
ROOT=/usr/local from the commandline if they want.

BUG=chromium-os:12474
TEST=Verify that gmerge works with this change, and that
     gmerge -n mesa works now (it previously didn't work because
     it depended on packages in package.provided).

Change-Id: I9f88fc87547578a8deff06d458274ed540a901d7
Reviewed-on: http://gerrit.chromium.org/gerrit/1214
Reviewed-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Reviewed-on: http://gerrit.chromium.org/gerrit/1232
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

coreos/dev-util,mgireesh05/dev-util,marineam/coreos-dev-util,mgireesh05/dev-util,mgireesh05/dev-util,marineam/coreos-dev-util,coreos/dev-util,coreos/dev-util