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/cm_fx6_defconfig b/configs/cm_fx6_defconfig
index 528251c..1a5b263 100644
--- a/configs/cm_fx6_defconfig
+++ b/configs/cm_fx6_defconfig
@@ -31,3 +31,4 @@
 CONFIG_USB_STORAGE=y
 CONFIG_USB_KEYBOARD=y
 CONFIG_SYS_USB_EVENT_POLL_VIA_INT_QUEUE=y
+CONFIG_OF_LIBFDT=y