Rover: replace arming-required with auto-arm-when-ready feature

[rmackay9]

Rover has an arming-required parameter which can be set to false allowing the vehicle to be in an armed state even though it hasn't passed the arming checks. I'm not a fan of this because it forces us to implement checks in other places to confirm the vehicle doesn't behave badly when it's in an unsafe state (e.g. Auto mode needs to double check that it has a position estimate). Implementing these checks here and there is error prone.

A better solution could be to remove the arming-required parameter and instead add an auto-arm feature which automatically arms the vehicle once it has passes arming checks.

This issue comes from this discussion