want UART methods to be virtual methods.

MbedSPI and MbedI2C methods are declared as virtual methods.
However the UART methods are not.

Since we want to create our own driver by inheriting from UART, it would be nice if the UART methods were also declared as virtual methods.

https://github.com/arduino/ArduinoCore-mbed/blob/1399d6477548f3f5585155652511d68d3ef85951/libraries/Wire/Wire.h#L34-L68

https://github.com/arduino/ArduinoCore-mbed/blob/1399d6477548f3f5585155652511d68d3ef85951/libraries/SPI/SPI.h#L28-L48

https://github.com/arduino/ArduinoCore-mbed/blob/1399d6477548f3f5585155652511d68d3ef85951/cores/arduino/Serial.h#L39-L67

Start with cores/arduino/Serial.h and compare its UART declarations with the virtual methods in libraries/Wire/Wire.h and libraries/SPI/SPI.h. Update the UART declarations so inherited drivers can override them, then verify the headers still compile and that the UART interface matches the requested behavior.

Written by the indexing model from the issue text.