makefiles/openocd: export OPENOCD_PRE_VERIFY_CMDS only when required
makefiles/openocd: export OPENOCD_PRE_VERIFY_CMDS only when required

kaspar030/RIOT,basilfx/RIOT,authmillenon/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,miri64/RIOT,ant9000/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,OTAkeys/RIOT,kaspar030/RIOT,authmillenon/RIOT,kaspar030/RIOT,authmillenon/RIOT,OTAkeys/RIOT,ant9000/RIOT,OTAkeys/RIOT,ant9000/RIOT,miri64/RIOT,kYc0o/RIOT,OTAkeys/RIOT,kYc0o/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,yogo1212/RIOT,miri64/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,basilfx/RIOT,yogo1212/RIOT,basilfx/RIOT,OlegHahm/RIOT,ant9000/RIOT,jasonatran/RIOT,ant9000/RIOT,yogo1212/RIOT,jasonatran/RIOT,OlegHahm/RIOT,kaspar030/RIOT,yogo1212/RIOT,basilfx/RIOT,yogo1212/RIOT,authmillenon/RIOT,kYc0o/RIOT,miri64/RIOT,jasonatran/RIOT,yogo1212/RIOT,basilfx/RIOT,miri64/RIOT,OTAkeys/RIOT