Guard against overheating when the firmware locks up

[dc42]

If the firmware locks up for any reason (e.g. a mains transient coupled through the USB cable ground) then the heaters become uncontrolled and overheating may result.

In the dc42 fork this has been fixed by (1) adding a tick interrupt routine that checks for overheat situations, (2) enabling the watchdog timer, and (3) kicking the watchdog during the tick interrupt.