fix bl_update: move definition of print_usage into the ifdef
fix bl_update: move definition of print_usage into the ifdef

fixes the compile error:
error: 'print_usage' defined but not used [-Werror=unused-function]

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