Adding IPv4 multicast support

[Colin Leitner]

> 
> I'll probably just push the patches to a github repository until picotcp lands.
> 

Patches can be found at

    https://github.com/cleitner/barebox-mcast/tree/multicast

Commits

  * 8e99783f775ef8013ea9f5e5f9fa881002cd21a2
  * f5f78253ce2e48affbb0b3bf9726b3b6e684b509
  * 0dd506b341a61855b9b526c6c0de3da2d4c4e0a9

are small cleanups to the network code and could be merged even if multicast support is deemed unnecessary.

Multicast connections are used like any other UDP connection:

    con = net_udp_new(string_to_ip("224.0.0.123"), 12345, handler, priv);
    net_udp_bind(con, 12345);

I only patched the drivers smc91111 (tested) and smc911x (untested) to receive multicast packets. Other drivers are likely just as easy to patch.

Have fun,
	Colin