boards/microbit: use qemu as default emulator
boards/microbit: use qemu as default emulator

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