Added serial port as parameter to launch files (Issue https://github.com/turtlebot/turtlebot/issues/111)

Allows to set the environmental variable TURTLEBOT_SERIAL_PORT that is used as serial port in all launch files (TODO: kobuki/mobile_base.launch.xml).

	modified:   create/mobile_base.launch.xml
	modified:   kobuki/mobile_base.launch.xml
	modified:   mobile_base.launch.xml
	modified:   roomba/mobile_base.launch.xml
	modified:   ../minimal.launch

Committer: mayrjohannes <joh.mayr@jku.at>
Author: mayrjohannes <joh.mayr@jku.at>