boards/hifive1b: fix default serial port
boards/hifive1b: fix default serial port

The board provides a JLink adapter which is exposed as /dev/ttyACMx on Linux. STDIO UART is available on this port

jasonatran/RIOT,x3ro/RIOT,yogo1212/RIOT,ant9000/RIOT,RIOT-OS/RIOT,x3ro/RIOT,smlng/RIOT,OlegHahm/RIOT,aeneby/RIOT,aeneby/RIOT,x3ro/RIOT,mtausig/RIOT,authmillenon/RIOT,aeneby/RIOT,x3ro/RIOT,toonst/RIOT,jasonatran/RIOT,OTAkeys/RIOT,jasonatran/RIOT,ant9000/RIOT,ant9000/RIOT,josephnoir/RIOT,kaspar030/RIOT,smlng/RIOT,kYc0o/RIOT,toonst/RIOT,jasonatran/RIOT,basilfx/RIOT,toonst/RIOT,OTAkeys/RIOT,kYc0o/RIOT,kaspar030/RIOT,OlegHahm/RIOT,smlng/RIOT,OlegHahm/RIOT,basilfx/RIOT,OTAkeys/RIOT,miri64/RIOT,miri64/RIOT,miri64/RIOT,josephnoir/RIOT,OTAkeys/RIOT,kaspar030/RIOT,toonst/RIOT,kaspar030/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,aeneby/RIOT,authmillenon/RIOT,aeneby/RIOT,mtausig/RIOT,yogo1212/RIOT,yogo1212/RIOT,authmillenon/RIOT,yogo1212/RIOT,smlng/RIOT,josephnoir/RIOT,miri64/RIOT,ant9000/RIOT,mtausig/RIOT,mtausig/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,josephnoir/RIOT,basilfx/RIOT,jasonatran/RIOT,josephnoir/RIOT,OlegHahm/RIOT,miri64/RIOT,toonst/RIOT,basilfx/RIOT,OTAkeys/RIOT,ant9000/RIOT,RIOT-OS/RIOT,smlng/RIOT,kaspar030/RIOT,authmillenon/RIOT,mtausig/RIOT,basilfx/RIOT,x3ro/RIOT,authmillenon/RIOT,kYc0o/RIOT,OlegHahm/RIOT,yogo1212/RIOT,authmillenon/RIOT