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/basecmd.h

@@ -9,7 +9,7 @@ void move_request_size(int size);
 void *oid_lookup(uint8_t oid, void *type);
 void *oid_alloc(uint8_t oid, void *type, uint16_t size);
 void *oid_next(uint8_t *i, void *type);
-void stats_note_sleep(uint32_t sleep_time);
+void stats_update(uint32_t start, uint32_t cur);
 
 #define foreach_oid(pos,data,oidtype)                   \
     for (pos=-1; (data=oid_next(&pos, oidtype)); )