Added ability for users to define a default serial target

I'd like to use arduino-cmake to always be able to start a serial connection with my arduino from within vim by binding a key to :!make serial. It must have a constant name, so I've added the ability to define a target simply called "serial" with the option DEFAULT_SERIAL for TARGET_NAME, which is the same command as "${TARGET_NAME}-serial". No idea if this is a feature you'd like but I just threw it together and thought I'd contribute back. Thanks for the quality work by the way! It's allowed me to free myself of the Arduino IDE.