Automatically set mqtt buffer size (#637)

* Automatically set mqtt buffer size

Current versions of pubsubclient allows the buffer size to be set
dynamically: https://github.com/knolleary/pubsubclient/pull/282

We can take advantage of this to avoid having to modify one of the
dependencies and instead automatically up the buffer size when a
powerful board is detected.

docs/use/ir.md

@@ -104,14 +104,12 @@ mosquitto_pub -t home/OpenMQTTGateway/commands/MQTTtoIR -m '{"raw":"8850,4450,60
 With big raw array you may cross the limit of default payload size. In this case the gateway will not receive the message or will not send it to the broker.
 In this case the best way is to use hex values instead, but if you can't you may change the parameters below:
 In user_config.h replace:
-`#define JSON_MSG_BUFFER 512`
+`# define JSON_MSG_BUFFER 512`
+`# define mqtt_max_packet_size 1024
 by
-`#define JSON_MSG_BUFFER 1280`
+`# define JSON_MSG_BUFFER 1280`
+`# define mqtt_max_packet_size 1280`
 
-And into platformio.ini
-`MQTT_MAX_PACKET_SIZE=1024`
-by
-`MQTT_MAX_PACKET_SIZE=1280`
 
 ## Repeat the IR signal OpenMQTTGateway receive
 So as to repeat the IR signal received by the gateway once set the following parameter to true in [config_IR.h](https://github.com/1technophile/OpenMQTTGateway/blob/091b317660fd201a30e2cd0e15424a13c5a6bd71/config_IR.h#L37)