platform/minimal-net: Implement better idle behavior.

The minimal-net target, as currently written, wakes up the
CPU every millisecond to check for packets, and will only
react in real-time to input from stdin. If you are running
this on a laptop battery, your battery will quickly drain.

This change allows the CPU to idle when there is literally
nothing to do while still being responsive to input from
stein and/or incoming packets. This fix should significantly
improve performance while significantly improving power
usage. Win-win.

Also added `_xassert()` implementation so that the contiki-
provided `assert()` macro will work properly when used
on this platform.

cpu/native/net/tapdev.c

@@ -69,6 +69,13 @@ static unsigned long lasttime;
 
 #define BUF ((struct uip_eth_hdr *)&uip_buf[0])
 
+/*---------------------------------------------------------------------------*/
+int
+tapdev_fd(void)
+{
+  return fd;
+}
+
 /*---------------------------------------------------------------------------*/
 static void
 remove_route(void)