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/mt_ventoux_defconfig b/configs/mt_ventoux_defconfig
index a0153e4..338df22 100644
--- a/configs/mt_ventoux_defconfig
+++ b/configs/mt_ventoux_defconfig
@@ -8,3 +8,4 @@
 CONFIG_CMD_GPIO=y
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_SYS_NS16550=y
+CONFIG_OF_LIBFDT=y