Fix symlink issue when generating an ARM image

A few symlinks were pointing to a nonexistent file named '(null)'
instead of their intended target. This was only seen when using the
arm_sdimage.sh script.

There is two ways of specifying links and directories. the first one
using an entry in one of the mtree files, and the second one in
/etc/Makefile.

Those entries where doubled, and one of them would specify some rights,
while the other had the target.

By removing those entries, I make sure there is only one definition of
these symlinks, which solves the problem I was seeing on an ARM image.

These symlinks are still present on a generated system, as they are
required.

Change-Id: I9ced8860f72d7c4d686a09720de4d8257d6e04fa

etc/mtree/NetBSD.dist.base

@@ -159,11 +159,3 @@
 # this one is for term(1)
 /set type=dir uid=0 gid=5 mode=775
 ./usr/spool/locks
-
-# Symlinks
-/set type=link uid=0 gid=0 mode=0777
-./dev/mouse
-./usr/bin/cc
-./usr/bin/strip
-./var/log
-./var/tmp