px4_base.cmake: make sure px_update_git_header.py is executed in the source directory
px4_base.cmake: make sure px_update_git_header.py is executed in the source directory

otherwise the 'git describe' command can fail.

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