As Audi's Quattro all-wheel-drive system celebrates its 40th anniversary, an RS 6 Avant has been decked out in tribute to one of Quattro's greatest motorsports moments. The Audi RS 6 GTO concept wears the racing livery of the Audi 90 IMSA GTO race car, which only competed in the 1989 season. The 90 GTO made…