Just to let you all know,

my first self compiled kernel runs fine, no major problems!
And some observations:

 - I compiled ext2fs support in the kernel and... was able
   to mount my Linux/68k ext2 filesystems! But I had to swap byteorder
   first (under Linux, using e2fsck -S), making it big endian. I think
   this is a bug of the NetBSD kernel: It should be able to read/write
   native (little endian) ext2fs even on a big endian machine like
   the atari...

 - installboot does not work anymore without patch: I had to comment
   out the OS version check to make it work... And, I didn't have to
   set boot preference to 'netbsd' to actually boot the kernel! Why is
   installboot trying to change boot preference? (I commented out
   the call to the switch procedure).
 
 - there was an unresolved symbol when linking the kernel:

     kern/kern_subr.c: _dumpspec 

   after defining it with const char *dumpspec = "none", the kernel
   was linked successfully.

Benni