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_O2M112_defconfig b/configs/O2MNT_O2M112_defconfig
index 5342719..3bf3cff 100644
--- a/configs/O2MNT_O2M112_defconfig
+++ b/configs/O2MNT_O2M112_defconfig
@@ -3,3 +3,4 @@
 CONFIG_TARGET_O2MNT=y
 CONFIG_SYS_EXTRA_OPTIONS="IFM_SENSOR_TYPE=\"O2M112\""
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_LIBFDT=y