Adding HASH_CHECK for onboard parameters

[basil-huber]

QGC introduced (silently) a way to avoid sending all parameters in the beginning.
The idea is to compute a hash over all parameter names and their values and to send it to QGC.
(QGC actually sends a request for the onboard parameter _HASH_CHECK).
If the _HASH_CHECK reveals no changes for the onboard parameters, they do not have to be sent and the parameters of the last session are loaded in QGC.

Here's the pull request of QGC: mavlink/qgroundcontrol#1965
Here's some thing from ardupilot: ArduPilot/ardupilot#3070
And here for PX4/Firmware: PX4/PX4-Autopilot#2982

Note: This is not required, it would just save some communication in the beginning