sntp callback: use a recurrent schedule function (esp8266#6888)

cores/esp8266/Schedule.h

@@ -46,7 +46,7 @@ void run_scheduled_functions();
 
 // recurrent scheduled function:
 //
-// * Internal queue may not be a FIFO.
+// * Internal queue is a FIFO.
 // * Run the lambda periodically about every <repeat_us> microseconds until
 //   it returns false.
 // * Note that it may be more than <repeat_us> microseconds between calls if
@@ -60,6 +60,7 @@ void run_scheduled_functions();
 //   recurrent function.
 // * If alarm is used, anytime during scheduling when it returns true,
 //   any remaining delay from repeat_us is disregarded, and fn is executed.
+
 bool schedule_recurrent_function_us(const std::function<bool(void)>& fn,
     uint32_t repeat_us, const std::function<bool(void)>& alarm = nullptr);
 

cores/esp8266/sntp-lwip2.cpp

@@ -455,15 +455,15 @@ int settimeofday(const struct timeval* tv, const struct timezone* tz)
         // apparently tz->tz_dsttime is a bitfield and should not be further used (cf man)
         sntp_set_daylight(0);
     }
-    if (tv) /* after*/
+    if (tv) /*after*/
     {
         // reset time subsystem
         tune_timeshift64(tv->tv_sec * 1000000ULL + tv->tv_usec);
 
         sntp_set_system_time(tv->tv_sec);
 
         if (_settimeofday_cb)
-            schedule_function(_settimeofday_cb);
+            schedule_recurrent_function_us([](){ _settimeofday_cb(); return false; }, 0);
     }
     return 0;
 }