navigator: fix takeoff jump edge case
navigator: fix takeoff jump edge case

In the normal sitl `commander takeoff` case, the takeoff jump was never
actually carried out because the default altitude radius is set to 3m
and the takeoff altitude to ~2m which means that the takeoff waypoint is
"reached" immediately.

This works around this edge case by using the altitude between the home
altitude and takeoff altitude divided by 2 as a acceptance radius.

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