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/tapdev6.c

@@ -85,6 +85,13 @@ static unsigned long lasttime;
 static void do_send(void);
 uint8_t tapdev_send(uip_lladdr_t *lladdr);
 
+/*---------------------------------------------------------------------------*/
+int
+tapdev_fd(void)
+{
+  return fd;
+}
+
 
 uint16_t
 tapdev_poll(void)
@@ -161,6 +168,7 @@ tapdev_init(void)
   /*  gdk_input_add(fd, GDK_INPUT_READ,
       read_callback, NULL);*/
 
+  atexit(&tapdev_exit);
 }
 /*---------------------------------------------------------------------------*/
 static void