sys: xtimer: replace XTIMER_MASK with XTIMER_WIDTH in board config

boards/airfy-beacon/include/board.h

boards/arduino-mega2560/include/board.h

boards/cc2650stk/include/board.h

boards/chronos/include/board.h

boards/fox/include/board.h

boards/iotlab-common/include/board_common.h

boards/msb-430-common/include/board_common.h

boards/nrf51dongle/include/board.h

boards/nucleo-f103/include/board.h

boards/pca10000/include/board.h

boards/pca10005/include/board.h

boards/spark-core/include/board.h

boards/telosb/include/board.h

boards/waspmote-pro/include/board.h

boards/wsn430-common/include/board_common.h

boards/yunjia-nrf51822/include/board.h

boards/z1/include/board.h

sys/include/xtimer.h