ppc4xx: Fix in-tree build for ppc440-generic boards

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@uam.es>
Signed-off-by: Stefan Roese <sr@denx.de>

board/xilinx/ppc440-generic/config.mk

@@ -22,4 +22,4 @@
 # MA 02111-1307 USA
 #
 
-sinclude $(obj)/board/$(BOARDDIR)/config.tmp
+sinclude $(OBJTREE)/board/$(BOARDDIR)/config.tmp