Makefile.features: warning if CPU not defined by BOARD/Makefile.features
Makefile.features: warning if CPU not defined by BOARD/Makefile.features

CPU must be defined by `$(RIOTBOARD)/$(BOARD)/Makefile.features` or
one of its common included Makefile.features file.

This currently only adds a warning but not prevents building for the
moment.

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