introduce sys_timeouts_sleeptime (returns the time left before the next timeout is due, for NO_SYS==1)

src/core/timers.c

@@ -470,7 +470,7 @@ sys_check_timeouts(void)
         }
       }
     /* repeat until all expired timers have been called */
-    }while(had_one);
+    } while(had_one);
   }
 }
 
@@ -485,6 +485,18 @@ sys_restart_timeouts(void)
   timeouts_last_time = sys_now();
 }
 
+/** Return the time left before the next timeout is due. If no timeouts are
+ * enqueued, returns 0xffffffff
+ */
+u32_t
+sys_timeouts_sleeptime(void)
+{
+  if (next_timeout == NULL) {
+    return 0xffffffff;
+  }
+  return (sys_now() - timeouts_last_time) + next_timeout->time;
+}
+
 #else /* NO_SYS */
 
 /**