service: retrieve lcore active state

service: retrieve lcore active state

This commit adds a new experimental API which allows the user
to retrieve the active state of an lcore. Knowing when the service
lcore is completed its polling loop can be useful to applications
to avoid race conditions when e.g. finalizing statistics.

The service thread itself now has a variable to indicate if its
thread is active. When zero the service thread has completed its
service, and has returned from the service_runner_func() function.

Suggested-by: Lukasz Wojciechowski <l.wojciechow@partner.samsung.com>
Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Reviewed-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>