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>