sched: Implement generic sleep mechanism based on tasks pending

Track when tasks are pending and spin in irq_wait() when no tasks are
pending.  This improves the mechanism for sleeping the processor -
it's simpler for the board specific code and it reduces the
possibility of the processor sleeping when tasks are busy.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/sched.h

@@ -30,6 +30,7 @@ void sched_add_timer(struct timer*);
 void sched_del_timer(struct timer *del);
 unsigned int sched_timer_dispatch(void);
 void sched_wake_tasks(void);
+uint8_t sched_tasks_busy(void);
 void sched_wake_task(struct task_wake *w);
 uint8_t sched_check_wake(struct task_wake *w);
 uint8_t sched_is_shutdown(void);