makefiles/dfu.inc.mk: use FLASHFILE
makefiles/dfu.inc.mk: use FLASHFILE

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

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