riotboot: use epoch as APP_VER default
riotboot: use epoch as APP_VER default

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