add JSON parsing and hardware serial support to RS232 gateway

[mreijnde]

Description:

I added some features to the RS232 gateway I like to share. My use-case is logging JSON data to MQTT generated by an other microprocessor through a serial connection to OMG running on a ESP8266 or ESP32.

Improvements:

• support for hardware serial (instead of software serial) to support higher baud rates
• support parsing of JSON received on the serial link to separate data in MQTT (sub) topics based on the JSON keys (up to the given nesting level).

EXAMPLE:
retrieved data on serial link: "{temperature: {sens1: 22, sens2: 23}, humidity: {sens1: 80, sens2: 60}}"

output to MQTT ( RS232maxJSONlevel=1):
./RS232toMQTT/temperature ==> "{sens1: 22, sens2: 23}"
./RS232toMQTT/humidity ==> "{sens1: 80, sens2: 60}"

output to MQTT ( RS232maxJSONlevel=2 (or higher)):
./RS232toMQTT/temperature/sens1 ==> 22
./RS232toMQTT/temperature/sens2 ==> 23
./RS232toMQTT/humidity/sens1 ==> 80
./RS232toMQTT/humidity/sens2 ==> 60

As default the original behavior is enabled to send the full RS232 like as MQTT message. By setting the RS232toMQTTmode to 1, the new JSON parsing mode can be enabled.

I tested it on a ESP32 and a ESP8266. The original 'raw' mode compiles on a Arduino Uno, the JSON variant does not fit in flash. The Arduino Mega does not compile, like on the original branch. I removed reference to it in the update.

Please have a look , I am open to suggestions to improve.

Checklist:

• The pull request is done against the latest development branch
• Only one feature/fix was added per PR and the code change compiles without warnings
• I accept the DCO.

[1technophile]

@DieKatzchen would it be possible to review this PR as you are the author of the first version ?

[DieKatzchen]

Gave it a look over, only thing I would change is putting the SoftwareSerial #include inside the #ifndef, since we wouldn't be using it in that instance. The serial stuff all looks about right (actually, I learned a thing or two), and I can't comment on the JSON formatting.

[1technophile]

@mreijnde Thanks for the PR, could you take into account the comment please

[mreijnde]

I moved the #include as requested.

Furthermore:

• I added short explanation to the RS232 document
• Reverted the commit that removed pin config for Arduino Mega. Compile error earlier was due a misconfiguration.

[1technophile]

Thanks!