the line in etc/Makefile:
	distrib-dirs:
		cd ${DESTDIR}/; rm -f ${DESTDIR}/sys; ln -s usr/src/sys sys
assumes it exists, and points at it.

*** old/src/etc/mtree/BSD.usr.dist	Thu Sep 30 23:03:14 1993
--- new/src/etc/mtree/BSD.usr.dist	Sun Jan 23 20:31:17 1994
***************
*** 154,157 ****
--- 154,159 ----
  	..
  ..
  src type=dir uname=bin gname=bin mode=0755
+ 	sys type=dir uname=bin gname=bin mode=0755
+ 	..
  ..