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

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