tools/avrdude.inc.mk: use FLASHFILE variable
tools/avrdude.inc.mk: use FLASHFILE variable

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

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