fdt: Add warning about CONFIG_OF_EMBED

This option has crept into use with some boards. Add a warning to try to
prevent this.

As an example:
   https://lists.denx.de/pipermail/u-boot/2017-September/304966.html



Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>