Use of #define LWIP_SINGLE_NETIF 1 by default on Pico W

For minimalism reasons it might be useful to enable by default the LWIP_SINGLE_NETIF in lwipopts.h.

At the moment the SDK does not compile with that option.

[peterharperuk]

I can't see any issues in the SDK. It seems to be a problem in pico-examples and the cyw43-driver.
raspberrypi/pico-examples#514
georgerobotics/cyw43-driver#124