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/mx23evk_defconfig b/configs/mx23evk_defconfig
index 55e8915..43e3d0a 100644
--- a/configs/mx23evk_defconfig
+++ b/configs/mx23evk_defconfig
@@ -7,3 +7,4 @@
 # CONFIG_CMD_SETEXPR is not set
 # CONFIG_CMD_NET is not set
 # CONFIG_CMD_NFS is not set
+CONFIG_OF_LIBFDT=y