In current firmware in printer.cpp around line 1400 you see
#endif // DRIVE_SYSTEM
Extruder::selectExtruderById(0);
#if FEATURE_SERVO // set servos to neutral positions at power_up
There extruder 0 gets selected for start. Set this to 2 and it starts with that extruder. Just be aware that hosts will maybe not see that 2 is active. Our server/host will detect it from message issued when printer resets during connect.