Set KERNOBJDIR and KERNSRCDIR (they're distinct from BSDOBJDIR and
BSDSRCDIR).

A sample from my /etc/mk.conf:

BSDSRCDIR=/u3/NetBSD-current/src
BSDOBJDIR=/usr/obj
KERNOBJDIR=/usr/obj/kernobjs/arch/${MACHINE}/compile
KERNSCRDIR=	${BSDSRCDIR}/sys
MKOBJDIRS=yes
RELEASEDIR=/u5/newrelease

Note, however, that you may need to tweak install media (floppy) image
creation to use this stuff.  I had some problems there that I was trying
to fix; some other changes happened in the i386 stuff which I think were
similar but I haven't tried playing with them yet.

-- 
==John Kohl <jtk@kolvir.arlington.ma.us>, <john_kohl@alum.mit.edu>
Home page: <http://people.ne.mediaone.net/jtk/>
Bicycling and Skiing to keep fit.