boards/*: make brief description consistent

boards/airfy-beacon/include/board.h

boards/b-l072z-lrwan1/include/board.h

boards/b-l475e-iot01a/include/board.h

boards/calliope-mini/include/board.h

boards/cc2650stk/include/board.h

boards/ek-lm4f120xl/include/board.h

boards/f4vi1/include/board.h

boards/feather-m0/include/board.h

boards/fox/include/board.h

boards/frdm-k22f/include/board.h

boards/frdm-k64f/include/board.h

boards/iotlab-a8-m3/include/board.h

boards/iotlab-m3/include/board.h

boards/limifrog-v1/include/board.h

boards/maple-mini/include/board.h

boards/microbit/include/board.h

boards/mips-malta/include/board.h

boards/msbiot/include/board.h

boards/mulle/include/board.h

boards/nrf51dongle/include/board.h