fdt: Introduce helper method fdt_overlay_apply_verbose()

Introduce fdt_overlay_apply_verbose, a method that applies an
overlay but in the case of an error produces a helpful message.

In addition if a base tree is found to be missing the __symbols__
node the message will point out that the probable reason is that
the base tree was miscompiled without the -@ option.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Acked-by: Simon Glass <sjg@chromium.org>