build: allow additional flags to be passed via EXTRAFLAGS
build: allow additional flags to be passed via EXTRAFLAGS

this allows for flags needed for the APM build

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