You also want this -> DKWEDGE_METHOD_BSDLABEL as well as (or even
instead of) DKWEDGE_METHOD_MBR. That cause a wedge to be added for each
disklabel entry which should the system to find your root filesystem.
It appears that with just DKWEDGE_METHOD_MBR the system didn't notice
where its root fs was at. In fact, you may need to leave out
DKWEDGE_METHOD_MBR and just use DKWEDGE_METHOD_BSDLABEL if your
disklabel contains information about all of the native NetBSD
filesystems and the linux ones.
You will have to change everything in your /etc/fstab to use /dev/dkN
notation rather than /dev/wd0M notation, but that isn't too hard to do.