[rotorcraft] add MODE_STARTUP to make flying without RC possible

- MODE_STARTUP defaults to AP_MODE_KILL (as before), but can be set to AP_MODE_NAV
- init nav, guidance and stabilization from autopilot_init
- after init of the above, set the autopilot_mode so it propagates to the guidance submodes.
- also call autopilot_init after the ahrs_init for ahrs_is_aligned check...
- Added check if autopilot_motors_on do determine if you want to switch to AP_MODE_FAILSAFE
[ardrone2] Parrot GPS show sattelite information.

closes #474