[rotorcraft] force MODE_STARTUP instead of KILL until ahrs is aligned

- Go to MODE_STARTUP (which defaults to AP_MODE_KILL) instead of AP_MODE_KILL as long as the ahrs is not aligned.
- Remove AUTOPILOT_DISABLE_AHRS_KILL from all example airframes (but kept the functionality for now).

MODE_STARTUP is set to AP_MODE_NAV for the autonomous (without RC) ARDrone2.
Without AUTOPILOT_DISABLE_AHRS_KILL it will force MODE_STARTUP until the ahrs is aligned,
but still not allow to turn on the motors (as that also checks for ahrs_is_aligned).

As far as I can see this should solve #964 as long as you don't have AUTOPILOT_DISABLE_AHRS_KILL defined.
[rotorcraft] arming: only check if motors can be armed if ahrs_is_aligned
[messages] possibility to add descriptions
Merge pull request #983 from paparazzi/ahrs_kill

[rotorcraft] force MODE_STARTUP instead of KILL until ahrs is aligned

- Go to MODE_STARTUP (which defaults to AP_MODE_KILL) instead of AP_MODE_KILL as long as the ahrs is not aligned.
- Remove AUTOPILOT_DISABLE_AHRS_KILL from all example airframes (but kept the functionality for now).

MODE_STARTUP is set to AP_MODE_NAV for the autonomous (without RC) ARDrone2.
Without AUTOPILOT_DISABLE_AHRS_KILL it will force MODE_STARTUP until the ahrs is aligned,
but still not allow to turn on the motors (as that also checks for ahrs_is_aligned).

This closes #964 (as long as you don't have AUTOPILOT_DISABLE_AHRS_KILL defined).
[modules][messages] mission: LLA waypoints in 1e7deg instead of float

A 32bit float has 23bits for the mantissa, which is a bit more than 7 digits.
Since you already use 3 digits to represent the part to the left of the decimal point, that leaves about 4 digits for the rest..
So in the worst case of 180deg you have a resolution of only ~3m!

With 1e7deg 32bit int you get ~1cm resolution.

addresses first part of #984 (at least for the rotorcraft version, fixedwing version still converts to LLA float in between...)
[modules] mission_rotorcraft: only check for ned_initialized_i