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

lib/pubsubclient/src/PubSubClient.h

@@ -26,7 +26,7 @@
 	#if defined(ESP8266) || defined(ESP32) || defined(__AVR_ATmega2560__) || defined(__AVR_ATmega1280__)
 	  #define MQTT_MAX_PACKET_SIZE 1024
 	#else // boards with smaller memory
-	  #define MQTT_MAX_PACKET_SIZE 256
+	  #define MQTT_MAX_PACKET_SIZE 128
 	#endif
 #endif