Add function etimer_reset_set.

This new function is similar to reset, but allows to also set a new
timeout. Thus long-term accuracy with changing timeouts is now possible.

core/sys/etimer.c

@@ -181,6 +181,14 @@ etimer_set(struct etimer *et, clock_time_t interval)
 }
 /*---------------------------------------------------------------------------*/
 void
+etimer_reset_with_new_interval(struct etimer *et, clock_time_t interval)
+{
+  timer_reset(&et->timer);
+  et->timer.interval = interval;
+  add_timer(et);
+}
+/*---------------------------------------------------------------------------*/
+void
 etimer_reset(struct etimer *et)
 {
   timer_reset(&et->timer);

core/sys/etimer.h

@@ -114,6 +114,19 @@ CCIF void etimer_set(struct etimer *et, clock_time_t interval);
  */
 CCIF void etimer_reset(struct etimer *et);
 
+/**
+ * \brief      Reset an event timer with a new interval.
+ * \param et   A pointer to the event timer.
+ * \param interval The interval before the timer expires.
+ *
+ *             This function very similar to etimer_reset. Opposed to
+ *             etimer_reset it is possible to change the timout.
+ *             This allows accurate, non-periodic timers without drift.
+ *
+ * \sa etimer_reset()
+ */
+void etimer_reset_with_new_interval(struct etimer *et, clock_time_t interval);
+
 /**
  * \brief      Restart an event timer from the current point in time
  * \param et   A pointer to the event timer.