cmake remove need for platform px4_add_board_os

boards/aerotenna/ocpoc/ubuntu.cmake

boards/airmind/mindpx-v2/default.cmake

boards/atlflight/eagle/default.cmake

boards/atlflight/excelsior/default.cmake

boards/atlflight/excelsior/qurt-default.cmake

boards/auav/esc35-v1/default.cmake

boards/auav/x21/default.cmake

boards/beaglebone/blue/cross.cmake

boards/beaglebone/blue/native.cmake

boards/bitcraze/crazyflie/default.cmake

boards/emlid/navio2/cross.cmake

boards/emlid/navio2/native.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