Calibration update for xmega and board support tweaks

    * config/avr/boards/xmegaa3buxplained/boardsupport.c: Initialise pin
        after clock
    * config/avr/boards/xmegaa3buxplained/calibration: Correct
        calibration for blowfish and dijkstra
    * config/avr/chips/atxmega256a3bu/chipsupport.h: Clear the port
        before initialising direction