Add Leonardo-like led pulse when bootloader is active

[facchinm]

Use a naive way to produce a led "pulse", no pwm, only systick and unsigned arithmetics 😄
Based on top of #64

[ArduinoBot]

✅ Build completed.

⬇️ Build URL: http://downloads.arduino.cc/PR/samd/package_samd-b46_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 Leonardo-like led pulse when bootloader is active #69
5. Select one of the boards under SAMD Pull Request Add Leonardo-like led pulse when bootloader is active #69 in Tools->Board menu
6. Compile/Upload as usual

[cmaglie]

Superseded by #165