Cut the 'stay awake' timer down to 20 seconds.

This is mostly for testing purposes, as waiting 50 seconds for the
system to sleep is tedious. Need to determine what the final
‘production’ value should be.
Avoid spending any more time than absolutely necessary with the watchdog off during startup.

Also explicitly disable the LIN protocol engine before trying to sleep
the board. This *may* help the situation where we try to sleep while
LIN activity is happening.

With these changes in place, have not been able to reproduce #97