UART: restore default buffer size to 256 bytes

[facchinm]

Fixes #650

[ArduinoBot]

✅ Build completed.

⬇️ Build URL: http://downloads.arduino.cc/PR/samd/package_samd-b215_index.json

ℹ️ To test this build:

1. Open the Preferences of the Arduino IDE.
2. Add the Build URL above in the Additional Boards Manager URLs field, and click OK.
3. Open the Boards Manager (menu Tools->Board->Board Manager...)
4. Install Arduino SAMD core - Pull Request UART: restore default buffer size to 256 bytes #651
5. Select one of the boards under SAMD Pull Request UART: restore default buffer size to 256 bytes #651 in Tools->Board menu
6. Compile/Upload as usual

[maxgerhardt]

But then the buffer size is again non-user-overridable?

ArduinoCore-samd/cores/arduino/Uart.h

Lines 25 to 29 in a5d52db

	#ifdef SERIAL_BUFFER_SIZE
	#undef SERIAL_BUFFER_SIZE
	#endif
	#define SERIAL_BUFFER_SIZE 256