makefiles/riotboot.mk: slots only need to depend on BASELIBS
makefiles/riotboot.mk: slots only need to depend on BASELIBS

Using 'link' was working too but will introduce a circular dependency
when FLASHFILE is one of the slot files.

This trims down to the minimal required dependency to work. It is now
the same as `ELFFILE` dependencies.

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