fix make O=dir build

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

scripts/test_make_O

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+b=`basename $PWD`
+test "${b#busybox}" != "$b" || { echo "Must be run in busybox tree"; exit 1; }
+
+rm -rf ../testdir_make_O.$$
+mkdir ../testdir_make_O.$$
+odir=`cd ../testdir_make_O.$$ && pwd`
+test -d "$odir" || exit 1
+
+make O="$odir" $MAKEOPTS "$@" defconfig busybox 2>&1 | tee test_make_O.log