Change how PCM bootloader determines IP address

The PCM firmware bootloader uses multicast, which requires that the sending host (the BEE, or similar) to know its IP address. In our world we have multiple interfaces, some with multiple networks; there is no obvious POSIX scheme to determine the right endpoint for a given remote IP address.

I had used a Linux-y mechanism, but Debian 8 broke that. I will switch to using the python netifaces package. Sigh.