sched: Move timer dispatch loop to board code

Rename sched_timer_kick() to sched_timer_dispatch() and move its loop
into its callers in the board code.  This eliminates the need to
export timer_try_set_next() from the board code.

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

src/generic/misc.h

@@ -13,7 +13,6 @@ uint32_t timer_from_us(uint32_t us);
 uint8_t timer_is_before(uint32_t time1, uint32_t time2);
 uint32_t timer_read_time(void);
 void timer_periodic(void);
-uint8_t timer_try_set_next(unsigned int next);
 
 size_t alloc_maxsize(size_t reqsize);