NuttX integration merge

Don't zero out the reported airspeed when below 15km/h

Multirotor attitude controller updated

Kalman navigation fixes

Added global position to logging

Fixed yaw estimate, minor comment and code style improvements. Added option for upfront init, prepared process for removal (later) of sensors app and subscription to individual topics. Prototyping rework of params / subscriptions to resolve GCC 4.7 incompatibility of control lib (or rather the fact that we need to work around something which looks like a compiler bug)

sdlog2: FLOW message added