I believe the following patch to sys/arch/hp300/stand/Makefile solves the 
problem.

*** Makefile-dist	Wed Feb 22 05:23:09 1995
--- Makefile	Tue Jul 11 13:25:04 1995
***************
*** 24,29 ****
--- 24,30 ----
  
  INCPATH=-I${.CURDIR} -I${.CURDIR}/../.. -I${S} -I${S}/lib/libsa
  
+ KERNREL=
  ### find out what to use for libkern
  .include "$S/lib/libkern/Makefile.inc"
  LIBKERN=	${KERNLIB}
***************
*** 123,134 ****
  
  # utilities
  
! clean cleandir:
  	rm -f *.o *.exe *.i errs make.out
  	rm -f a.out pboot cat ls tboot tcopy
  	rm -f boot[a-z]? boot[a-wyz][a-z].c conf[a-wyz][a-z].c
  	rm -f core sboot
  	rm -f libdrive.a mkboot dboot dcopy installboot *.lif
  
  install: mkboot installboot ${ALL}
  	./mkboot pboot netbsdboot.lif
--- 124,137 ----
  
  # utilities
  
! clean::
  	rm -f *.o *.exe *.i errs make.out
  	rm -f a.out pboot cat ls tboot tcopy
  	rm -f boot[a-z]? boot[a-wyz][a-z].c conf[a-wyz][a-z].c
  	rm -f core sboot
  	rm -f libdrive.a mkboot dboot dcopy installboot *.lif
+ 
+ cleandir: clean
  
  install: mkboot installboot ${ALL}
  	./mkboot pboot netbsdboot.lif