Old crossdev would symlink its metadata dir to the chromiumos overlay.
New crossdev manages this itself iff it had created the layout.conf
before. Add an upgrade hook to convert the symlink over to a config
file that crossdev will update.
BUG=chromium-os:26998
TEST=`./update_chroot` with old chroot created metadata overlay that crossdev would update
Change-Id: I03faa78a988e0cd0a94dcd4f0f01151c32bb215b
CQ-DEPEND=I3f2775111da800622591c9b56ba00428d6106207
Reviewed-on: https://gerrit.chromium.org/gerrit/17408
Reviewed-by: Zdenek Behan <zbehan@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
