post: Fix building with O=

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
diff --git a/post/board/lwmon5/Makefile b/post/board/lwmon5/Makefile
index 3cb6426..4e95515 100644
--- a/post/board/lwmon5/Makefile
+++ b/post/board/lwmon5/Makefile
@@ -20,7 +20,7 @@
 # along with this program; if not, write to the Free Software
 # Foundation, Inc., 59 Temple Place, Suite 330, Boston,
 # MA 02111-1307 USA
-include $(TOPDIR)/include/autoconf.mk
+include $(OBJTREE)/include/autoconf.mk
 
 LIB	= libpostlwmon5.a