BOD threshold now 1.8v with external clock option

The BOD threshold has been lowered from 2.7v to 1.8v when using the
external clock option in config.h.

This change prevents BOD from resetting the clock prematurely when the
external clock might have still been able to keep time.  Even so, the
new setting will also allow time loss during sleep when the battery
gets low.  But if users notice time loss after a power outage, they
should know to change the battery anyway.