makefiles/vars.inc.mk: update doc on CPU/CPU_MODEL definition
makefiles/vars.inc.mk: update doc on CPU/CPU_MODEL definition

Part of moving CPU/CPU_MODEL definition to Makefile.features to have it
available before Makefile.include.

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