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/sama5d3xek_nandflash_defconfig b/configs/sama5d3xek_nandflash_defconfig
index 1e42880..968cd66 100644
--- a/configs/sama5d3xek_nandflash_defconfig
+++ b/configs/sama5d3xek_nandflash_defconfig
@@ -8,3 +8,4 @@
 # CONFIG_CMD_FPGA is not set
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_ATMEL=y
+CONFIG_OF_LIBFDT=y