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/stm32f429-discovery_defconfig b/configs/stm32f429-discovery_defconfig
index fb330e2..00d105b 100644
--- a/configs/stm32f429-discovery_defconfig
+++ b/configs/stm32f429-discovery_defconfig
@@ -4,3 +4,4 @@
 CONFIG_TARGET_STM32F429_DISCOVERY=y
 CONFIG_SYS_PROMPT="U-Boot > "
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_LIBFDT=y