Some memory optimization for Arduino UNO following this thread:

https://community.openmqttgateway.com/t/arduino-uno-ethernet-shield-reboots-on-receiving-rf-signal-on-v-0-9/346/22
Rework led management to avoid use of global variables
and specify special JSON_MSG_BUFFER for arduino UNO
led can now be used for all the boards

generalize use of F macro
for memory optimization
change jsonbuffer calculation method
reduce pubsubclient max packet size for uno and nano

ZgatewayRF.ino

@@ -47,7 +47,8 @@ void setupRF(){
 void RFtoMQTT(){
 
   if (mySwitch.available()){
-    StaticJsonBuffer<JSON_MSG_BUFFER> jsonBuffer;
+    const int JSON_MSG_CALC_BUFFER = JSON_OBJECT_SIZE(4);
+    StaticJsonBuffer<JSON_MSG_CALC_BUFFER> jsonBuffer;
     JsonObject& RFdata = jsonBuffer.createObject();
     trc(F("Rcv. RF"));
     #ifdef ESP32
@@ -61,10 +62,6 @@ void RFtoMQTT(){
     RFdata.set("delay", (int)mySwitch.getReceivedDelay());
     mySwitch.resetAvailable();
     
-    trc(F("LED MNG"));
-    digitalWrite(led_receive, LOW);
-    timer_led_receive = millis();
-    
     unsigned long MQTTvalue = RFdata.get<unsigned long>("value");
     if (!isAduplicate(MQTTvalue) && MQTTvalue!=0) {// conditions to avoid duplications of RF -->MQTT
         #ifdef ZmqttDiscovery //component creation for HA