dist/tools/build_system_sanity_check: deprecate FEATURES_MCU_GROUP
dist/tools/build_system_sanity_check: deprecate FEATURES_MCU_GROUP

Add a function to list deprecated variables or patterns and use it for
* FEATURES_MCU_GROUP

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