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.
Now tcpip_output() is a function pointer that is supposed to be set via the macro tcpip_set_outputfunc(). Packet drivers do so on process startup.
Thus if there are several packet drivers in a Contiki system the one started last is the one actually used. This behaviour is especially useful for the 'IP forwarding' "meta" packet driver.
