[settings] Allow settings blocks to specify a sibling ordering

Allow settings blocks to provide an explicit default ordering between
siblings, with lower precedence than the existing ${priority} setting.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/settings.c

 		tmp_priority = fetch_intz_setting ( tmp, &priority_setting );
 		if ( priority > tmp_priority )
 			break;
+		if ( settings->order > tmp->order )
+			break;
 	}
 	list_add_tail ( &settings->siblings, &tmp->siblings );
 

src/include/ipxe/settings.h

 	struct settings_operations *op;
 	/** Default scope for numerical settings constructed for this block */
 	const struct settings_scope *default_scope;
+	/** Sibling ordering */
+	int order;
 };
 
 /**