FERGUSON, MO., Nov 26 (Reuters) - Since looting first erupted following the August police shooting of black teenager Michael Brown, nearly all the businesses in a 2 square mile area of this St Louis suburb have had to board up. All except one - a Conoco gas station and convenience store. At least a dozen stores have been set ablaze and others looted in Ferguson in racially charged riots since a grand jury on Monday cleared white policeman Darren Wilson in the shooting, which has torn apart this predominantly black Missouri city. The unrest surrounding Brown's death has underscored the often-tense nature of U.S.