HAL_PX4: include visibility.h

needed for new nuttx structure
HAL_PX4: moved rc.APM to new ROMFS directory
PX4: added base ROMFS files from upstream
PX4: added in remaining example mixers
build: added support for the new PX4 build system
build: fixed build_binaries.sh for new PX4 build
PX4: auto-build the archives if needed
build: added px4-archives target
PX4: changed EXTRAFLAGS to EXTRADEFINES for PX4