Change uptime calculation method (#901)

and revert #897

main/ZmqttDiscovery.ino

@@ -174,7 +174,7 @@ void pubMqttDiscovery() {
   createDiscovery("sensor", //set Type
                   subjectSYStoMQTT, "SYS: Uptime", (char*)getUniqueId("uptime", "").c_str(), //set state_topic,name,uniqueId
                   "", "", "{{ value_json.uptime }}", //set availability_topic,device_class,value_template,
-                  "", "", "min", //set,payload_on,payload_off,unit_of_meas,
+                  "", "", "s", //set,payload_on,payload_off,unit_of_meas,
                   0, //set  off_delay
                   "", "", true, "", //set,payload_avalaible,payload_not avalaible   ,is a gateway entity, command topic
                   "", "", "", "", false // device name, device manufacturer, device model, device mac, retain

main/main.ino

@@ -1318,12 +1318,26 @@ void loop() {
 #endif
 }
 
+/** 
+ * Calculate uptime and take into account the millis() rollover
+ */
+unsigned long uptime() {
+  static unsigned long lastUptime = 0;
+  static unsigned long uptimeAdd = 0;
+  unsigned long uptime = millis() / 1000 + uptimeAdd;
+  if (uptime < lastUptime) {
+    uptime += 4294967;
+    uptimeAdd += 4294967;
+  }
+  lastUptime = uptime;
+  return uptime;
+}
+
 #if defined(ESP8266) || defined(ESP32) || defined(__AVR_ATmega2560__) || defined(__AVR_ATmega1280__)
 void stateMeasures() {
   StaticJsonBuffer<JSON_MSG_BUFFER> jsonBuffer;
   JsonObject& SYSdata = jsonBuffer.createObject();
-  unsigned long uptime = millis() / 60000; //minutes uptime
+  SYSdata["uptime"] = uptime();
-  SYSdata["uptime"] = uptime;
   SYSdata["version"] = OMG_VERSION;
   Log.trace(F("retrieving value of system characteristics Uptime (min):%u" CR), uptime);
 #  if defined(ESP8266) || defined(ESP32)