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/sam3x8e/timer.c

@@ -18,13 +18,6 @@ timer_set(uint32_t value)
     TC0->TC_CHANNEL[0].TC_RA = value;
 }
 
-// Return the next scheduled wake up time
-uint32_t
-timer_get_next(void)
-{
-    return TC0->TC_CHANNEL[0].TC_RA;
-}
-
 // Return the current time (in absolute clock ticks).
 uint32_t
 timer_read_time(void)