mpu6000: allow for two mpu6000 instances, one internal, one external
mpu6000: allow for two mpu6000 instances, one internal, one external

split g_dev into g_dev_int and g_dev_ext

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