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/at91sam9261ek_nandflash_defconfig b/configs/at91sam9261ek_nandflash_defconfig
index d7a98c6..6a5a850 100644
--- a/configs/at91sam9261ek_nandflash_defconfig
+++ b/configs/at91sam9261ek_nandflash_defconfig
@@ -10,3 +10,4 @@
 # CONFIG_CMD_FPGA is not set
 # CONFIG_CMD_SOURCE is not set
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_LIBFDT=y