stmhal: Add network and usocket module.
As per issue #876, the network module is used to configure NICs (hardware modules) and configure routing. The usocket module is supposed to implement the normal Python socket module and selects the underlying NIC using routing logic. Right now the routing logic is brain dead: first-initialised, first-used. And the routing table is just a list of registered NICs. cc3k and wiznet5k work, but not at the same time due to C name clashes (to be fixed). Note that the usocket module has alias socket, so that one can import socket and it works as normal. But you can also override socket with your own module, using usocket at the backend.