mini-os: build fixes for lwip 1.3.2

mini-os: build fixes for lwip 1.3.2

Various fixes to mini-os needed to build lwip 1.3.2:

- Don't build the tests.
- Add BSD-style endianness macros to endian.h.
- free() is called via a function pointer so it needs to be a real
  function.  Do the same for malloc() and realloc().

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Committed-by: Keir Fraser <keir@xen.org>