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/x600_defconfig b/configs/x600_defconfig
index 71dc7dc..881c080 100644
--- a/configs/x600_defconfig
+++ b/configs/x600_defconfig
@@ -10,3 +10,4 @@
 CONFIG_NETDEVICES=y
 CONFIG_ETH_DESIGNWARE=y
 CONFIG_USE_TINY_PRINTF=y
+CONFIG_OF_LIBFDT=y