stateful_update: rename var to var_overlay
stateful_update: rename var to var_overlay

Expect to find var_overlay now instead of var, due to
Ie6480a59929818fe5d36a46abf533b648fb78850.

BUG=chromium-os:22172
TEST=None
CQ-DEPEND=Ie6480a59929818fe5d36a46abf533b648fb78850

Change-Id: I1f386a3365eada999d4bd2ca588e82c24ba3caf8
Signed-off-by: Kees Cook <50cb10d2be062154b17ab4d66a4eab34f1cf1c84@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/26622
Reviewed-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>

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