Add basic network streaming of audio over UDP.

The audio output of the receiver can now be streamed to a spefied host and
port using UDP. Currently only one channel is streamed using 48 kHz sample
rate. The audio is converted to short (S16LE) and a client side command
line could be:

  nc -l -u 7355 | aplay -r 48k -f S16_LE -t raw -c 1