I have had the same problem with all recent versions of NetBSD
and the PC I have as the console.  When the console reboots, some
where in its hardware reset sequence, the serial ports send a
BREAK.  I have used some old (perhaps non standard) laptops   
which do not do the same.  

A break has nothing to do with the DTR line.  A break is formed
by holding the TX line inactive for a second or two.  Zero voltage
is not inactive, so pulling the cable out of the port doesn't
send a break.  The inactive signal is a voltage between 3-20 volts
or so (and I can't remember if it is positive or negative).  The
active signal is the same voltage, but opposite polarity. 

Anyway, getting back to the point, I would like to add an option
in the kernel which will ignore the console break.  I looked thru
the source code a long time ago, and didn't see any good way of
doing this except by commenting out the call to the debugger.

Any other suggestions???????????