doc/porting-boards: fix broken links
doc/porting-boards: fix broken links

ant9000/RIOT,RIOT-OS/RIOT,ant9000/RIOT,basilfx/RIOT,jasonatran/RIOT,kYc0o/RIOT,yogo1212/RIOT,OlegHahm/RIOT,yogo1212/RIOT,ant9000/RIOT,yogo1212/RIOT,kaspar030/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,ant9000/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,OlegHahm/RIOT,basilfx/RIOT,miri64/RIOT,authmillenon/RIOT,kaspar030/RIOT,jasonatran/RIOT,authmillenon/RIOT,ant9000/RIOT,authmillenon/RIOT,kYc0o/RIOT,yogo1212/RIOT,yogo1212/RIOT,jasonatran/RIOT,kYc0o/RIOT,OTAkeys/RIOT,basilfx/RIOT,miri64/RIOT,OTAkeys/RIOT,miri64/RIOT,OTAkeys/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,miri64/RIOT,kaspar030/RIOT,jasonatran/RIOT,OlegHahm/RIOT,authmillenon/RIOT,jasonatran/RIOT,yogo1212/RIOT,basilfx/RIOT,OlegHahm/RIOT,miri64/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,OTAkeys/RIOT,basilfx/RIOT,kYc0o/RIOT