Use SAMD21 serial number for USB serial number prefix

[sandeepmistry]

This is based on section 9.3.3 of the data sheet.

With this change, Blink is 1948 bytes larger on a MKR1000 (without change 9256 bytes, with change 11204 bytes).

Anyone know of a slimmer way to convert a register value to a hex string?

[sandeepmistry]

I've made things slimmer using something based on utoa with leading zero padding and upper case conversion.

Blink on a MKR1000 is now 9760 bytes, without the patch it's 9256 bytes - so now a 504 byte increase.

[sandeepmistry]

Even simpler now with @cmaglie's suggestion, now Blink is down to 9336 bytes, so just an 80 byte increase!

[ArduinoBot]

✅ Build completed.

⬇️ Build URL: http://downloads.arduino.cc/PR/samd/package_samd-b158_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 Use SAMD21 serial number for USB serial number prefix #252
5. Select one of the boards under SAMD Pull Request Use SAMD21 serial number for USB serial number prefix #252 in Tools->Board menu
6. Compile/Upload as usual