MBED SerialN UART drivers are unbuffered in transmit path

[WestfW]

All MBED SerialN UART drivers are unbuffered in the transmit path, which means that they block pretty much all the time. This is undesirable.

Related: https://forum.arduino.cc/t/uart-serial-write-appears-to-be-blocking-arduino-mbed-rasberry-pi-pico/1290268 https://forum.arduino.cc/t/serialx-x-1-4-are-unbuffered-surprised/1178363 https://github.com/arduino/ArduinoCore-mbed/issues/272

[megacct]

Hi, I raised #763 last year but received no response. I have converted my code to use mbed::BufferedSerial