Add TX buffer for Serial

[sandeepmistry]

Resolves #33

[facchinm]

@ArduinoBot build this please

[chamnit]

I'm curious why this pull request has stalled. I implemented these changes into IDE 1.6.12 and SAMD core 1.6.11. It seems to work and looks similar to the ASF's usart_interrupt.c code that does the same thing there.

[ArduinoBot]

✅ Build completed.

⬇️ Build URL: http://downloads.arduino.cc/PR/samd/package_samd-b154_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 Add TX buffer for Serial #58
5. Select one of the boards under SAMD Pull Request Add TX buffer for Serial #58 in Tools->Board menu
6. Compile/Upload as usual