I just committed some patches which make KGDB
work much better:
-attach at early system startup (same time as DDB)
-single stepping works now
-if ^C in gdb issues a line break, the system is stopped
  and debugger interaction is possible
  ("set remotebreak")

To enable KGDB support, put "options KGDB"
inti the configuration file. The device defaults to
com0, speed 19600.
The device can be changed by option "KGDB_DEVNAME"
(currently only "com1" is possible), the speed by option
"KGDBRATE".
The old option "KGDBDEV" does not work anymore -
it doesn't make much sense to speak about major and minor
numbers in this context anyway.

There are still fixes necessary to drop into DDB on a serial
console on line break. I'll do this another day...

best regards
Matthias Drochner