This patch allows to use lwIP as the implementation of the usocket module,
instead of the existing socket-multiplexer that delegates the entire TCP/IP
layer to the NIC itself.
This is disabled by default, and enabled by defining MICROPY_PY_LWIP to 1.
When enabled, the lwIP TCP/IP stack will be included in the build with
default settings for memory usage and performance (see
lwip_inc/lwipopts.h). It is then up to a particular NIC to register itself
with lwIP using the standard lwIP netif API.
