Canonicalize correctly even if PREFIX does not exist

The existence test is done on next line, and with -f in place,
canonicalization or non-existent PREFIX leads to empty PREFIX which
makes the next message confusing

utils/mkit/include/mkit.sh

     sed -e 's/ = /=/' < config.mk > $tmp
     . $tmp
     rm -f $tmp
-    PREFIX="$(readlink -f "$PREFIX")"
+    PREFIX="$(readlink -m "$PREFIX")"
     test -d "$PREFIX" || die "PREFIX points to bad directory: $PREFIX"
 }