dist/tools/buildsystem_sanity_check: check no PREFLASH* exports
dist/tools/buildsystem_sanity_check: check no PREFLASH* exports

Add sanity check for removed exports.

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