SHANGHAI, Aug 30 (Reuters) - Nio (9866.HK) plans to launch its first self-developed mobile telephone in late September, the Chinese electric vehicle maker said, aiming to improve the appeal of its cars with better software and connectivity. It becomes China's latest automaker to join a trend of rolling out car systems that allow drivers to use their smartphones to remotely open doors, turn on air-conditioning or start their cars, among other functions. Nio's founder and chief executive, William Li, was a key driver of the phone project, which the automaker is targeting mainly at drivers of its cars. "Our phone...