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/advanced-configuration.md

@@ -0,0 +1,34 @@
+# Advanced configuration of the gateway
+
+## Secure connection to the broker
+By default the connection between the gateway and the MQTT broker is not encrypted and the identity of the broker is not verified.
+For connections in a private local area network this might not be a big issue, but if you connect to a broker on the internet or the gateway is connected to a public network there is a high security risk.
+
+To secure the connection, Transport Layer Security (TLS) can be used which is supported by most MQTT brokers.
+Setting up and configuring TLS in the MQTT broker is a complex process and perhaps include creating a self-signed certificate.
+The configuration of the broker is not covered here, you should look into the documentation of your broker.
+
+### Prerequisites
+The MQTT broker is configured for TLS and you have access to the CA certificate which was used to sign the MQTT broker certificate.
+You are using ESP8266 or ESP32, for other boards TLS is not supported.
+
+### Configure secure connection in the gateway
+To enable the secure connection and use TLS uncomment `//#define SECURE_CONNECTION` in `User_config.h`.
+Set `MQTT_SERVER` to the Common Name (CN) of the certificate of the broker.
+This can be the hostname or the ip of the broker.
+
+The CA certificate should be in PEM ascii format.
+If your CA certificate is not in the correct format or you don't know the format, use `openssl` to convert the certificate to the correct format.
+In `User_config.h` replace the `...` with the content of your certificate which is between the `-----BEGIN CERTIFICATE-----` and `-----END CERTIFICATE-----` lines:
+```cpp
+const char* certificate CERT_ATTRIBUTE = R"EOF("
+-----BEGIN CERTIFICATE-----
+...
+-----END CERTIFICATE-----
+")EOF";
+```
+
+If you have no ntp server in your local network (included in the router) or not using dhcp, you should uncomment `//#    define NTP_SERVER "pool.ntp.org"` to use a ntp server for time synchronization.
+This is related to the `W: failed, ssl error code=54` error message, which indicate that the time of the esp is not correct.
+
+You can know compile and upload to your board and the gateway should connect with TLS to your broker.