kernel: Add k_reschedule()

The routine k_reschedule() allows an application to manually force
a schedule point. Although similar to k_yield(), it has different
properties. The most significant difference is that k_yield() if
invoked from a cooperative thread will voluntarily give up execution
control to the next thread of equal or higher priority while
k_reschedule() will not.

Applications that play with EDF deadlines via k_thread_deadline_set()
may need to use k_reschedule() to force a reschedule.

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