sched: add a packing level knob

Date	Fri, 18 Oct 2013 13:52:18 +0200

The knob is used to set an average load threshold that will be used to trig
the inclusion/removal of CPUs in the packing effort list.

Signed-off-by: Vincent Guittot <vincent.guittot at linaro.org>
Signed-off-by: flar2 <asegaert@gmail.com>