usb: Adapt error message to newly present build variables
usb: Adapt error message to newly present build variables

ant9000/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,jasonatran/RIOT,kYc0o/RIOT,kaspar030/RIOT,miri64/RIOT,miri64/RIOT,OTAkeys/RIOT,authmillenon/RIOT,miri64/RIOT,authmillenon/RIOT,kaspar030/RIOT,OlegHahm/RIOT,ant9000/RIOT,OTAkeys/RIOT,ant9000/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,authmillenon/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,authmillenon/RIOT,kYc0o/RIOT,OlegHahm/RIOT,jasonatran/RIOT,ant9000/RIOT,kYc0o/RIOT,authmillenon/RIOT,authmillenon/RIOT,kaspar030/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,kYc0o/RIOT,OlegHahm/RIOT,jasonatran/RIOT,jasonatran/RIOT,OTAkeys/RIOT,miri64/RIOT,RIOT-OS/RIOT,miri64/RIOT,kYc0o/RIOT