Added etimer_next_to_expire function

Added the etimer_next_to_expire function as a superior version of
etimer_next_expiration_time; the major benefits being:

1. We return NULL if no timers remaining
2. we can use the timer module functions to determine if the timer
   is expired, how long until it expires etc.

Author: bkozak-scanimetrics

core/sys/etimer.c

@@ -254,6 +254,12 @@ etimer_next_expiration_time(void)
   return etimer_pending() ? next_expiration : 0;
 }
 /*---------------------------------------------------------------------------*/
+const struct timer *
+etimer_next_to_expire(void)
+{
+  return timerlist ? &(timerlist->timer) : NULL;
+}
+/*---------------------------------------------------------------------------*/
 void
 etimer_stop(struct etimer *et)
 {

core/sys/etimer.h

@@ -245,6 +245,17 @@ int etimer_pending(void);
  */
 clock_time_t etimer_next_expiration_time(void);
 
+/**
+ * \brief      Return the timer which is next to expire
+ * \return     Const pointer to timer which will next expire, or NULL if no
+ *             timers pending.
+ *
+ *             This function returns the timer of the etimer which will expire
+ *             next so that existing timer module functions can be used to
+ *             check when the timer will expire or if it has already expired.
+ */
+const struct timer *etimer_next_to_expire(void);
+
 #define etimer_lt(t0, t1, c)  timer_lt(&(t0->timer), &(t1->timer), c)
 #define etimer_lte(t0, t1, c) timer_lte(&(t0->timer), &(t1->timer), c)
 #define etimer_gt(t0, t1, c)  timer_gt(&(t0->timer), &(t1->timer), c)