dist/tools/buildsystem_sanity_check: add an export variable check
dist/tools/buildsystem_sanity_check: add an export variable check

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.

toonst/RIOT,authmillenon/RIOT,mtausig/RIOT,authmillenon/RIOT,jasonatran/RIOT,toonst/RIOT,rfuentess/RIOT,cladmi/RIOT,aeneby/RIOT,basilfx/RIOT,mfrey/RIOT,kaspar030/RIOT,smlng/RIOT,RIOT-OS/RIOT,ant9000/RIOT,jasonatran/RIOT,cladmi/RIOT,basilfx/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,miri64/RIOT,OlegHahm/RIOT,kaspar030/RIOT,aeneby/RIOT,ant9000/RIOT,ant9000/RIOT,yogo1212/RIOT,yogo1212/RIOT,miri64/RIOT,kYc0o/RIOT,cladmi/RIOT,mtausig/RIOT,ant9000/RIOT,kYc0o/RIOT,rfuentess/RIOT,mtausig/RIOT,x3ro/RIOT,aeneby/RIOT,mfrey/RIOT,cladmi/RIOT,kaspar030/RIOT,smlng/RIOT,RIOT-OS/RIOT,rfuentess/RIOT,miri64/RIOT,ant9000/RIOT,josephnoir/RIOT,aeneby/RIOT,smlng/RIOT,yogo1212/RIOT,mfrey/RIOT,jasonatran/RIOT,cladmi/RIOT,rfuentess/RIOT,kYc0o/RIOT,toonst/RIOT,toonst/RIOT,OTAkeys/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,x3ro/RIOT,basilfx/RIOT,yogo1212/RIOT,x3ro/RIOT,x3ro/RIOT,miri64/RIOT,josephnoir/RIOT,mfrey/RIOT,OTAkeys/RIOT,aeneby/RIOT,kYc0o/RIOT,authmillenon/RIOT,mfrey/RIOT,rfuentess/RIOT,josephnoir/RIOT,OTAkeys/RIOT,jasonatran/RIOT,kaspar030/RIOT,authmillenon/RIOT,authmillenon/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,smlng/RIOT,OlegHahm/RIOT,mtausig/RIOT,jasonatran/RIOT,smlng/RIOT,OlegHahm/RIOT,authmillenon/RIOT,yogo1212/RIOT,basilfx/RIOT,miri64/RIOT,kYc0o/RIOT,x3ro/RIOT,basilfx/RIOT,kaspar030/RIOT,mtausig/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,toonst/RIOT