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/net2big_v2_defconfig b/configs/net2big_v2_defconfig
index 87f4f92..c0aee31 100644
--- a/configs/net2big_v2_defconfig
+++ b/configs/net2big_v2_defconfig
@@ -8,3 +8,4 @@
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_MACRONIX=y
 CONFIG_SYS_NS16550=y
+CONFIG_OF_LIBFDT=y