added support for secure TLS connections (#699)

* added support for secure TLS connections
on ESP32 and ESP8266 close #101

* added TLS test builds

* added documentation for TLS
fixed some some issues in the documentation

* added documentation for error code=54

docs/upload/troubleshoot.md

@@ -62,3 +62,16 @@ This can be due to corruption of the ESP flash memory, try to erase flash and up
 
 If you didn't find your answer here post a question to the forum:
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+
+## ESP does not connect to broker with TLS enabled
+If you get the following error:
+`W: failed, ssl error code=54` ("Certificate is expired or not yet valid.")
+
+This is most probable caused by the time of the esp is not correct/synchronized.
+The esp uses the Network Time Protocol (NTP) to get the current time from a time server.
+If you get this error ntp is not configured correctly in the gateway.
+Uncomment `//#    define NTP_SERVER "pool.ntp.org"` to set the `pool.ntp.org` as the time server.
+You can also choose any other ntp time server you like.
+
+It is normal that the time synchronization process takes some time and the MQTT connection will not be successful the first time.
+If you set the ntp server for the gateway and keep getting the errors you should check your certificate validity duration.