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/at91sam9rlek_nandflash_defconfig b/configs/at91sam9rlek_nandflash_defconfig
index 220f95f..3e825aa 100644
--- a/configs/at91sam9rlek_nandflash_defconfig
+++ b/configs/at91sam9rlek_nandflash_defconfig
@@ -13,3 +13,4 @@
 # CONFIG_CMD_SETEXPR is not set
 # CONFIG_CMD_NET is not set
 # CONFIG_CMD_NFS is not set
+CONFIG_OF_LIBFDT=y