New auto-configuration scheme

[fwsGonzo]

Quoting @AndreasAakesson
This one is more explicit, and supports dhcp-with-fallback as needed for Acorn and Demo_Service #1456.

test/net/integration/configure/config.json:

{
  "net": [
    {
      "iface": 0,
      "config":  "static",
      "address": "10.0.0.42",
      "netmask": "255.255.255.0",
      "gateway": "10.0.0.1"
    },
    {
      "iface": 1,
      "config":  "static",
      "address": "10.0.0.43",
      "netmask": "255.255.255.0",
      "gateway": "10.0.0.1",
      "dns":     "8.8.8.8"
    },
    {
      "iface": 2,
      "config": "none"
    },
    {
      "iface": 3,
      "config": "dhcp"
    },
    {
      "iface": 5,
      "config": "dhcp-with-fallback",
      "address": "10.0.0.44",
      "netmask": "255.255.255.0",
      "gateway": "10.0.0.1"
    }
  ]
}