cmake px4_add_board document and cleanup

boards/aerotenna/ocpoc/ubuntu.cmake

boards/airmind/mindpx-v2/default.cmake

boards/atmel/same70xplained/default.cmake

boards/auav/esc35-v1/default.cmake

boards/auav/x21/default.cmake

boards/av/x-v1/default.cmake

boards/beaglebone/blue/cross.cmake

boards/bitcraze/crazyflie/default.cmake

boards/emlid/navio2/cross.cmake

boards/gumstix/aerocore2/default.cmake

boards/intel/aerofc-v1/default.cmake

boards/intel/aerofc-v1/rtps.cmake

boards/nxp/hlite-v3/default.cmake

boards/omnibus/f4sd/default.cmake

boards/parrot/bebop/default.cmake

boards/px4/cannode-v1/default.cmake

boards/px4/esc-v1/default.cmake

boards/px4/fmu-v2/default.cmake

boards/px4/fmu-v2/lpe.cmake

boards/px4/fmu-v2/test.cmake