Add support for RS485 on Controllino Maxi/Mega

[vzahradnik]

Support for RS485 on Controllino Maxi/Mega

• This PR adds support for RS485 integrated into Controllino Maxi/Mega boards.
• These boards use different PINs and different Serial object. Otherwise, the logic works the same.
• I added a simple #ifdef...endif clause to check for these boards.
• If users include the CONTROLLINO library, these flags are set for them. Including their library is recommended by their official guides. As an alternative, users can just set the flag #define CONTROLLINO_MAXI or #define CONTROLLINO_MEGA in their sketch. In such a case, I added a clause inside src/RS485.h, which tries to load the CONTROLLINO library.

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:white_check_mark: vzahradnik
:x: Robert-Faltus
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[pmmarquez]

We at Controllino are now updating our board support packages and we pretend to ad support for ArduinoRS485 by defining RS485_SERIAL_PORT, RS485_DEFAULT_TX_PIN, RS485_DEFAULT_RE_PIN, RS485_DEFAULT_DE_PIN we just need the small change on #46, anyway thanks to @vzahradnik for this effort 👍

[aentinger]

I am closing this PR, since the intended functionality has been achieved via merging #51 .