Specify `SERIAL_RX_BUFFER_SIZE` or similar define

`SERIAL_RX_BUFFER_SIZE` is a define from the legacy Arduino Cores.

- https://github.com/arduino/ArduinoCore-avr/blob/321fca0bac806bdd36af8afbc13587f4b67eb5f1/cores/arduino/HardwareSerial.h#L49
- https://github.com/arduino/ArduinoCore-megaavr/blob/5e639ee40afa693354d3d056ba7fb795a8948c11/cores/arduino/UART.h#L50

We use it to optimize large serial transfers, and it would be nice to see it specified in the ArduinoCore_mbed.

If you have a different / similar approach, that would also be very helpful.

Start by comparing the linked legacy headers, cores/arduino/HardwareSerial.h and cores/arduino/UART.h, to understand how SERIAL_RX_BUFFER_SIZE is exposed. Then locate the corresponding serial implementation in ArduinoCore-mbed and determine how a similar define can support large transfers; done means the buffer-size setting is specified and usable there.

Written by the indexing model from the issue text.