base empty implementation for non-pure virtual functions in HardwareSerial.h

[majbthrd]

Although the GCC 7-2017Q4 presently bundled into the Arduino SAMD v1.8.3 package doesn't complain, more recent compiler versions are not as accommodating. What I've seeing with GCC 8-2018Q4 is:

Uart.o: in function 'HardwareSerial::HardwareSerial()': undefined reference to 'vtable for HardwareSerial'

The Uart class is derived from HardwareSerial, and the compiler error is due to begin() and end() not being pure virtual and not having a base class implementation.

I see two options. One would be to declare begin() and end() as pure virtual (as everything declared in HardwareSerial is). However, I presume there is some latent reasoning by the author to exclude those particular declarations. If we change this, there also seems to be the possibility of breaking someone's code.

The second option would be to provide a base empty implementation for these functions. This satisfies the compiler's need to have a default implementation should the derived classes not implement one.

[aentinger]

I'm closing this since ArduinoCore-API was introduced to ArduinoCore-samd with #560. In ArduinoCore-API those functions in questions are in fact defined pure virtual, as was one of your suggested fixes.