Introduce MG_NET_IF var, replaces MG_NET_IF_*

Possible values:
  MG_NET_IF_SOCKET (1) - traditional BSD socket API
  MG_NET_IF_SIMPLELINK (2) - TI's SimpleLink socket interface
  MG_NET_IF_LWIP_LOW_LEVEL - a net_if implementation that uses LWIP's
    low-level API, when LWIP_SOCKET is not available.

PUBLISHED_FROM=a5c0aebb7b6bdb1f1f4cb3210763d16f3ab5e133