[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>

libfdt/fdt_strerror.c

@@ -16,9 +16,6 @@
  * License along with this library; if not, write to the Free Software
  * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
  */
-#include "config.h"
-#if CONFIG_OF_LIBFDT
-
 #include "libfdt_env.h"
 
 #include <fdt.h>
@@ -65,5 +62,3 @@ const char *fdt_strerror(int errval)
 
 	return "<unknown error>";
 }
-
-#endif /* CONFIG_OF_LIBFDT */