lwIP ifconfig_env
=================
The lwip_ifconfig_env library allows configuring a static IPv4 address
(including network prefix and gateway) or DHCP without modifying the network
application. It is linked as an additional library and the configuration is
specified using environment variables, e.g.:

    IFCONFIG_IP4_<iface>="192.168.42.2/24 via 192.168.42.1"
    IFCONFIG_IP4_<iface>="dhcp"

The network interface is configured early as part of static constructor
initialization. Note that the library currently does not wait for DHCP
configuration to finish, so a network client application may fail if it does
not wait until it has obtained an IP address. For network server applications
it usually does not matter when the IP is assigned - they bind to a port and
then just wait for incoming requests.
