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/vf610twr_nand_defconfig b/configs/vf610twr_nand_defconfig
index b6a96f2..4591526 100644
--- a/configs/vf610twr_nand_defconfig
+++ b/configs/vf610twr_nand_defconfig
@@ -14,3 +14,4 @@
 CONFIG_SYS_NAND_BUSWIDTH_16BIT=y
 CONFIG_SPI_FLASH=y
 CONFIG_FSL_LPUART=y
+CONFIG_OF_LIBFDT=y