Move CONFIG_OF_LIBFDT to Kconfig

Move this option to Kconfig and tidy up existing boards.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/mx6cuboxi_defconfig b/configs/mx6cuboxi_defconfig
index e8964d9..58e297a 100644
--- a/configs/mx6cuboxi_defconfig
+++ b/configs/mx6cuboxi_defconfig
@@ -8,3 +8,4 @@
 CONFIG_CMD_GPIO=y
 CONFIG_DM=y
 CONFIG_DM_THERMAL=y
+CONFIG_OF_LIBFDT=y