examples/default: fix for 802.15.4 radio devices

This adapts the shell command `txtsnd` for 802.15.4 capable radio devices.

**known issues**:
* source address only gets displayed correctly when payload is >=8 characters (?!?)
* payload length on receiver side is zero
`DEBUG_ENABLED` is used with `#if DEBUG_ENABLED` to easily switch additional output on and off besides `DEBUG()` (i.e. call `ieee802154_frame_print_fcf_frame()`).