shell: prepare a rudimentary IEEE 802.15.4 packet

The transceiver module expects an `ieee802154_packet_t` instead of a
`radio_packet_t` if the device supports the IEEE 802.15.4 packet format.
This commit fixes the corresponding transceiver shell command for
`txtsnd` to set destination address (short address mode), payload, and
length accordingly.
Merge pull request #1434 from OlegHahm/fix_txtsnd_ieee802.15.4

shell: prepare a rudimentary IEEE 802.15.4 packet