Feature/app watchdog

[m-mcgowan]

Provides a critical-priority thread that wakes up at a given timeout interval to see if the application has checked in.

API:

// declare a global instance
ApplicationWatchdog wd(timeout_milli_seconds, timeout_function_to_call, stack_size=512);

• if the application has not exited loop, or called Particle.process() within the given timeout, the watchdog calls the given timeout function.
• A default stack size of 512 is used for the thread. The stack can be made larger or smaller as needed.

Example:

ApplicationWatchdog wd(60000, System.reset);
// reset the system after 60 seconds if the application is unresponsive

The application watchdog requires interrupts to be active in order to function. Enabling the hardware watchdog in tandem with this is advised to detect when interrupts are not firing.