boards/openmote-b: remove OFLAGS
boards/openmote-b: remove OFLAGS

OFLAGS is set by the `.bin` and `.hex` rules no need to define it anymore.

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