create PX4 platform layer initialization helper (#11269)

CMakeLists.txt

boards/airmind/mindpx-v2/src/CMakeLists.txt

boards/airmind/mindpx-v2/src/init.c

boards/atmel/same70xplained/src/CMakeLists.txt

boards/atmel/same70xplained/src/init.c

boards/auav/esc35-v1/src/CMakeLists.txt

boards/auav/esc35-v1/src/init.c

boards/auav/x21/src/CMakeLists.txt

boards/auav/x21/src/init.c

boards/av/x-v1/src/CMakeLists.txt

boards/av/x-v1/src/init.c

boards/bitcraze/crazyflie/src/CMakeLists.txt

boards/bitcraze/crazyflie/src/init.c

boards/gumstix/aerocore2/src/CMakeLists.txt

boards/gumstix/aerocore2/src/init.c

boards/intel/aerofc-v1/src/CMakeLists.txt

boards/intel/aerofc-v1/src/init.c

boards/nxp/fmuk66-v3/src/CMakeLists.txt

boards/nxp/fmuk66-v3/src/init.c

boards/omnibus/f4sd/src/CMakeLists.txt