pixhawk-v1/2: using make px4_fmu_v3_default instead of make px4_fmu_v2_default

[xianglunkai]

I find we should update command for pixhawkv1/2 to compile and upload application program

[Hamish Willee]

Can you be precise about what boards where you would make the change? The original Pixhawk v1/v2 cannot run px4_fmu_v3 firmware so we can't, for example, update this section: http://docs.px4.io/main/en/flight_controller/pixhawk.html#building-firmware