Versioning of CPX protocol

[evoggy]

Currently there's no version on the CPX protocol and this should be added. The suggestion is to take 2 of the MSB from the function field, which would then become 6 bits. Since with the current implementation the 2 MSB are already 0 we can decide that the first version of the protocol is 0 and the implementation will not be effected. It's just the definition of the package that will change.

This versioning is not something we need to enforce yet, we can wait until we have a next version.

[knmcguire]

We need to start enforcing this before the next release. The version should be printed out and an assert should happen when a cpx version message comes in that is not the right version.

I'll make issues in https://github.com/bitcraze/aideck-gap8-examples, https://github.com/bitcraze/aideck-esp-firmware, and https://github.com/bitcraze/crazyflie-lib-python

[knmcguire]

Moreover, we should document the version as well