boards/lpc2k_pgm: use FLASHFILE for boards using lpc2k_pgm
boards/lpc2k_pgm: use FLASHFILE for boards using lpc2k_pgm

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

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