To use this feature, add the following to a sketch, for example
GatewayESP8266MQTTClient:
const uint8_t bssid[] = {0x00, 0xDE, 0xAD, 0xBE, 0xEF, 0x00};
#define MY_ESP8266_BSSID bssid
This feature was requested in
https://github.com/mysensors/MySensors/issues/947