mavlink_mission: support INT altitude frames
mavlink_mission: support INT altitude frames

The relative altitude flag was not set correclty when waypoints are sent
using the INT protocol.

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