kernel: extend thread runtime stats

When the new Kconfig option CONFIG_SCHED_THREAD_USAGE_ANALYSIS
is enabled, additional timing stats are collected during context
switches. This extra information allows a developer to obtain the
the current, longest, average and total lengths of the time that
a thread has been scheduled to execute.

A developer can in turn use this information to tune their app and/or
alter their scheduling policies.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>