Reworked airspeed driver to keep most of it abstract

makefiles/config_px4fmu-v1_default.mk

src/drivers/airspeed/airspeed.cpp

src/drivers/airspeed/airspeed.h

src/drivers/airspeed/module.mk

src/drivers/ets_airspeed/ets_airspeed.cpp

src/drivers/meas_airspeed/meas_airspeed.cpp

src/drivers/meas_airspeed/module.mk