I've tried this again, and it may be that the problems stem from the 
fact that I am doing this on a FreeBSD system.

I've managed to make boot using nbmake (Is there a way to do this from 
build.sh?) Then, on to mkboot. Using the host system's make did not 
work, I guess FreeBSD uses a different kind of make. Using nbmake 
works, after setting the mk include directory and setting 
HOST_CPPFLAGS='-nostdinc -I~/NetBSD-usr/include -I.' where 
NetBSD-usr/include are the includes from a NetBSD system. But then:

hppa/byte_swap.h: In function `__byte_swap_long':
hppa/byte_swap.h:44: unknown register name `r22' in `asm'

Is there any way to compile under FreeBSD 5.1, or should I just give up?

Kirill