[BUILD] conditionally compile libfdt/*.c in libfdt/Makefile

Modify libfdt/Makefile to conditionally compile the *.c files based
on the board config.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
diff --git a/libfdt/Makefile b/libfdt/Makefile
index 126fa2c..d166cce 100644
--- a/libfdt/Makefile
+++ b/libfdt/Makefile
@@ -27,7 +27,7 @@
 
 SOBJS	=
 
-COBJS-y += fdt.o fdt_ro.o fdt_rw.o fdt_strerror.o fdt_sw.o fdt_wip.o
+COBJS-$(CONFIG_OF_LIBFDT) += fdt.o fdt_ro.o fdt_rw.o fdt_strerror.o fdt_sw.o fdt_wip.o
 
 COBJS	:= $(COBJS-y)
 SRCS 	:= $(SOBJS:.o=.S) $(COBJS:.o=.c)