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/mx51evk_defconfig b/configs/mx51evk_defconfig
index f2c3743..0bb0c61 100644
--- a/configs/mx51evk_defconfig
+++ b/configs/mx51evk_defconfig
@@ -3,3 +3,4 @@
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/freescale/mx51evk/imximage.cfg"
 # CONFIG_CMD_IMLS is not set
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_LIBFDT=y