send MQTT heartbeat immediately on EMS-ESP boot so it can be detected by HA

[proddy]

There's an example HA automation script that checks the MQTT info topic to determine if EMS-ESP has booted. Problem is that it's a retained message on the MQTT broker so every time HA or the MQTT Server restarts it will be triggered.

I've update the automation script example to use the heartbeat and the key called bus_status is checked for a value called connecting which is only set once when EMS-ESP starts.

[proddy]

in 3.6.5-dev14