Pet watchdog during flash writes to avoid corrupted filesystem

[jepler]

@Neradoc noticed that it was easy to corrupt CIRCUITPY when the hardware watchdog is enabled. We would like to reduce the chance of a corrupt filesystem by regularly "feeding" the watchdog at short intervals during a flash write.

this doesn't fix all irritations about having watchdog enabled during development (e.g., maybe it bites during reload still even if it doesn't corrupt anything) but it would be a step in the right direction.

[deshipu]

I wonder how hard it would be to make a two-level watchdog for the users. We would basically have a usb-friendly watchdog implemented in C that would be friendly to all the filesystem and other needs, and that would be what the users interact with. But in addition, to guard against hard crashes, that "software" watchdog would in turn feed the hardware one, so if it ever stopped running, the board would restart hard (if the crash happened in the middle of a filesystem operation, the filesystem is already corrupted anyways).