Add Policy Platform vs Persistent parameters vs kbuild

[knmcguire]

Currently in the Crazyflie we have different platform files that can be made, compile options that can be enabled in kbuild and persistent parameters, which are all for configuring a new or variation of the platforms.

We need to document somewhere what the policy is on which variables should be what exactly what, as sometimes even we are confused about this 😄

[tobbeanton]

There is a good start in this file but it needs clarification of the policy.

[tobbeanton]

Suggestion now in PR #992

[krichardsson]

Fixed in #992