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/mx35pdk_defconfig b/configs/mx35pdk_defconfig
index c13e9f2..699d929 100644
--- a/configs/mx35pdk_defconfig
+++ b/configs/mx35pdk_defconfig
@@ -1,3 +1,4 @@
 CONFIG_ARM=y
 CONFIG_TARGET_MX35PDK=y
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_LIBFDT=y