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/generic/timer_irq.h

@@ -2,6 +2,5 @@
 #define __GENERIC_TIMER_IRQ_H
 
 uint32_t timer_dispatch_many(void);
-uint32_t timer_get_next(void);
 
 #endif // timer_irq.h