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/O2MNT_O2M113_defconfig b/configs/O2MNT_O2M113_defconfig
index bcbb598..05f6f08 100644
--- a/configs/O2MNT_O2M113_defconfig
+++ b/configs/O2MNT_O2M113_defconfig
@@ -3,3 +3,4 @@
 CONFIG_TARGET_O2MNT=y
 CONFIG_SYS_EXTRA_OPTIONS="IFM_SENSOR_TYPE=\"O2M113\""
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_LIBFDT=y