boards/opencm904: use FLASHFILE for boards using robotis-loader.py
boards/opencm904: use FLASHFILE for boards using robotis-loader.py

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

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