#25 Make lwipthread link status polling interval configurable

[Ignacio García Pérez]

The hardcoded 5 seconds interval is too large in some cases, and I, like most users, would rather not copy-and-modify the lwipthread implementation that comes with Chibi/OS.

This is the relevant code in lwipthread.c:

/* Setup event sources.*/
evtInit(&evt, S2ST(5));
evtStart(&evt);

[Giovanni Di Sirio]

Implemented.