fit: If no matching config is found in fit_find_config_node(), use the default one

If board_fit_config_name_match() doesn't match any configuration node,
then use the default one (if provided).

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