Create a custom open method for SHEARWATER family. This method can be used to pass a reference to a dc_serial_t structure. In this way the applications can implement their own implementation for a serial communication and set their callbacks for the basic serial functions. Signed-off-by: Claudiu Olteanu <olteanu.claudiu@ymail.com> Signed-off-by: Dirk Hohndel <dirk@hohndel.org>