boards/openocd: use FLASHFILE for boards using openocd
boards/openocd: use FLASHFILE for boards using openocd

Update to use FLASHFILE as file to be flashed on the board.

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