[FR] Support second UART to have other bitrate

[Teddyz]

Description

Make it possible to have SERIAL_PORT_2 using a new setting BAUDRATE_2

Feature Workflow

I think this code in MarlinCore.cpp will solve this:

  #if HAS_MULTI_SERIAL
    #if defined(BAUDRATE_2)
      MYSERIAL1.begin(BAUDRATE_2);
    #else
      MYSERIAL1.begin(BAUDRATE);
    #endif
    serial_connect_timeout = millis() + 1000UL;
    while (!MYSERIAL1 && PENDING(millis(), serial_connect_timeout)) { /*nada*/ }
  #endif

Configuration.h:

#define BAUDRATE 250000
#define BAUDRATE_2 9600

Additional Information

Some documentation in Configuration.h is needed.
I did some basic testing on my Mega2560-board (Trigorilla_14) and it seems to work, but I am no expert.

[thisiskeithb]

Implemented in #21949

[github-actions]

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