launch: add px4.launch script that only starts the px4 binary
launch: add px4.launch script that only starts the px4 binary

This provides a clean ROS interface for external projects which is
independent from the px4 CLI interface.

It can be used with:
```
	<include file="$(find px4)/launch/px4.launch">
	</include>
```

acfloria/Firmware,acfloria/Firmware,krbeverx/Firmware,PX4/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,Aerotenna/Firmware,Aerotenna/Firmware,Aerotenna/Firmware,dagar/Firmware,dagar/Firmware,acfloria/Firmware,krbeverx/Firmware,krbeverx/Firmware,acfloria/Firmware,PX4/Firmware,krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,krbeverx/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,Aerotenna/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,Aerotenna/Firmware,dagar/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,Aerotenna/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,Aerotenna/Firmware,dagar/Firmware,PX4/Firmware,PX4/Firmware