Getting closer: I need to use 
I2C3_DEV instead of 
I2C2 (CF 1.0). Now the firmware compiles without warnings, finally. Furthermore, I initialized 
I2C3_DEV a second time by mistake although it has already been initialized in 
imu_cf2.c. Fixed that.
The CF 2.0 can be found and connected with the PC Client, but telemetry data is a mess (weird values drifting without any evident correlation to the copter's movement, just as the ASL) and the artifical horizon doesn't respond at all. The console repeatedly outputs:
What does that mean, is there any error code information available? The copter works well with the standard firmware by the way.