Change the stxxst to CONFIG_OF_LIBFDT

This was configured to use the deprecated CONFIG_OF_FLAT_TREE, change
to CONFIG_OF_LIBFDT.

WARNING: It appears that this board lost its ability to boot via a
flattened device tree prior to this changeset.

WARNING: This conversion was untested because I do not have a board to
test it on.

Signed-off-by: Gerald Van Baren <vanbaren@cideas.com>

include/configs/stxxtc.h

@@ -590,8 +590,8 @@ typedef unsigned int led_id_t;
 
 /*****************************************************************************/
 
-/* pass open firmware flat tree */
-#define CONFIG_OF_FLAT_TREE	1
+/* pass open firmware flattened device tree */
+#define CONFIG_OF_LIBFDT	1
 
 #define OF_CPU			"PowerPC,MPC870@0"
 #define OF_TBCLK		(MPC8XX_HZ / 16)