Simple Watchdog Support

I would like to suggest an API to add simple watchdog support to the Arduino core. The default behavior could be to reset the board if locked up more than 5 seconds. The API could be implemented with the hardware watchdog or with a simple counter updated by the millis() ISR and timeout check in main(). Please see the below fork and latest commit for an implementation.

https://github.com/mikaelpatel/Arduino/commit/80cad523962a53632c4c31cd6e7a8ec62a422bad

Cheers!

Start by reviewing the proposed implementation in the linked fork commit 80cad523962a53632c4c31cd6e7a8ec62a422bad and the Arduino core entry points it changes. Compare the hardware-watchdog approach with the millis() ISR and main() timeout approach. Done means the core exposes simple watchdog support and resets a locked-up board after the intended timeout.

Written by the indexing model from the issue text.