examples/default: set src addr correctly
examples/default: fix handling of 802.15.4 devices
shell/transceiver: fix byte selection for 802.15.4 devices in output
sys/transceiver: cleanup and fix for cc2420 and at86rf231

cc2420: the payload pointer was set twice
at86rf231: payload_len was not copied from the radio driver buffer
Merge pull request #1674 from thomaseichinger/default_fix

examples/default:sys/transceiver: fix for 802.15.4 radio devices