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/mx23_olinuxino_defconfig b/configs/mx23_olinuxino_defconfig
index 22c46f8..4582cce 100644
--- a/configs/mx23_olinuxino_defconfig
+++ b/configs/mx23_olinuxino_defconfig
@@ -5,3 +5,4 @@
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_GPIO=y
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_LIBFDT=y