cpu/esp8266: use FLASHFILE variable
cpu/esp8266: use FLASHFILE variable

Use ELFFILE as FLASHFILE as all files are created from this
one using esptool.

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