calibrate_yaw.cfg should be renamed to imu.cfg

[KristofRobot]

According to http://wiki.ros.org/dynamic_reconfigure/Tutorials/HowToWriteYourFirstCfgFile, https://github.com/KristofRobot/razor_imu_9dof/blob/indigo-devel/config/calibrate_yaw.cfg has an issue:

NOTE: The third parameter should be equal to the cfg file name, without extension. Otherwise the libraries will be generated in every build, forcing a recompilation of the nodes which use them. 

I.e. we should either rename the cfg file to imu.cfg - or change following:

exit(gen.generate(PACKAGE, "razor_imu_9dof", "calibrate_yaw"))

I also think we should move this file to a cfg directory, rather than config, which is meant for yaml configuration files?