tests/riotboot: use 'RIOTBOOT_COMBINED_BIN' variable
tests/riotboot: use 'RIOTBOOT_COMBINED_BIN' variable

Use the new variable instead of the composed name.

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