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

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