ppc4xx: ppc440-generic-ALL: Fix out of tree build v2

This patch solves the problems compiling ml507, v5fx30teval and
ppc440-generic out of tree.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@uam.es>
diff --git a/board/xilinx/ppc440-generic/Makefile b/board/xilinx/ppc440-generic/Makefile
index f7405a8..bf0a6ba 100644
--- a/board/xilinx/ppc440-generic/Makefile
+++ b/board/xilinx/ppc440-generic/Makefile
@@ -35,9 +35,9 @@
 
 LIB	= $(obj)lib$(BOARD).a
 
-COBJS	+= $(SRCTREE)/board/xilinx/ppc440-generic/xilinx_ppc440_generic.o
+COBJS	+= ../../xilinx/ppc440-generic/xilinx_ppc440_generic.o
 
-SOBJS	+= $(SRCTREE)/board/xilinx/ppc440-generic/init.o
+SOBJS	+= ../../xilinx/ppc440-generic/init.o
 
 SRCS	:= $(SOBJS:.o=.S) $(COBJS:.o=.c)
 OBJS	:= $(addprefix $(obj),$(COBJS))