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/mx53ard_defconfig b/configs/mx53ard_defconfig
index 8f233c9..e8a211a 100644
--- a/configs/mx53ard_defconfig
+++ b/configs/mx53ard_defconfig
@@ -2,3 +2,4 @@
 CONFIG_TARGET_MX53ARD=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/freescale/mx53ard/imximage_dd3.cfg"
 # CONFIG_CMD_IMLS is not set
+CONFIG_OF_LIBFDT=y