sched: Support adding timers to the start of timer_list

If sched_add_timer() is called on a timer that would make it the new
head of the list, then add it and signal the board code that the timer
should be rescheduled.

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

src/generic/misc.h

@@ -11,6 +11,7 @@ void console_sendf(const struct command_encoder *ce, va_list args);
 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_kick(void);
 void timer_periodic(void);
 
 void *dynmem_start(void);