Check that some variables are not exported in the build system.
This should track variables that managed to not be exported anymore so
that they do not reappear in a BSP.
It is not a whitelist but just a way to keep things cleaned in the
future.
Add a script to execute sanity checks on build system files.
It should prevent bad patterns to re-appear after being cleaned.
Currently adds a check for using the content of `FEATURES` instead of
`USEMODULE`.
Modules should not check the content of FEATURES_PROVIDED/_REQUIRED/OPTIONAL
Handling specific behaviors/dependencies should by checking the content of:
* `USEMODULE`
* maybe `FEATURES_USED` if it is not a module (== not a periph_)
