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/mx6ul_14x14_evk_defconfig b/configs/mx6ul_14x14_evk_defconfig
index 835285f..77d78e7 100644
--- a/configs/mx6ul_14x14_evk_defconfig
+++ b/configs/mx6ul_14x14_evk_defconfig
@@ -6,3 +6,4 @@
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_PING=y
+CONFIG_OF_LIBFDT=y