change start script to launch the px4flow driver in background.
change start script to launch the px4flow driver in background.

Fixes issue #2145

mje-nz/PX4-Firmware,mcgill-robotics/Firmware,krbeverx/Firmware,Aerotenna/Firmware,darknight-007/Firmware,jlecoeur/Firmware,PX4/Firmware,jlecoeur/Firmware,mje-nz/PX4-Firmware,jlecoeur/Firmware,darknight-007/Firmware,mje-nz/PX4-Firmware,mcgill-robotics/Firmware,mcgill-robotics/Firmware,mcgill-robotics/Firmware,acfloria/Firmware,Aerotenna/Firmware,mcgill-robotics/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,darknight-007/Firmware,krbeverx/Firmware,jlecoeur/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,dagar/Firmware,dagar/Firmware,acfloria/Firmware,Aerotenna/Firmware,krbeverx/Firmware,krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,mcgill-robotics/Firmware,mje-nz/PX4-Firmware,darknight-007/Firmware,PX4/Firmware,dagar/Firmware,dagar/Firmware,dagar/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,Aerotenna/Firmware,acfloria/Firmware,PX4/Firmware,jlecoeur/Firmware,darknight-007/Firmware,Aerotenna/Firmware,dagar/Firmware,acfloria/Firmware,PX4/Firmware,mcgill-robotics/Firmware,PX4/Firmware,krbeverx/Firmware,jlecoeur/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,acfloria/Firmware