[scripts] add send_udp.py for certification (#840)
[scripts] add send_udp.py for certification (#840)

This commit adds script/reference-device/send_udp.py for certification.
openthread/borderrouter,openthread/ot-br-posix,openthread/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,openthread/borderrouter,openthread/ot-br-posix,openthread/borderrouter,openthread/borderrouter,openthread/ot-br-posix,openthread/borderrouter,openthread/ot-br-posix