KHARTOUM, May 20 (Reuters) - Sudan’s central bank is launching a system of foreign currency auctions as part of its flexible but managed floatation of the Sudanese pound, it said in a statement on Thursday. Sudanese banks and exchange bureaus have bought foreign currency worth $1.2 billion since a devaluation in February that brought the official rate in line with the black market, it added. (Reporting by Khalid Abdelaziz; Writing by Nafisa Eltahir; Editing by Andrew Cawthorne) Our Standards: The Thomson Reuters Trust Principles.