Import lwIP contrib rep

... from http://git.savannah.gnu.org/cgit/lwip/lwip-contrib.git/ into contrib/ subdir, STABLE-2_1_0_RELEASE tag
lwIP contrib is now officially frozen
TODO: Fix build

contrib/ports/unix/lib/README

@@ -0,0 +1,28 @@
+This directory contains an example of how to compile lwIP as a shared library
+on Linux.
+
+Some brief instructions:
+
+* Compile the code:
+
+ > mkdir build
+ > cd build
+ > cmake ..
+ > make clean all
+
+ This should produce liblwip.so. This is the shared library.
+
+* Link an application against the shared library
+
+ If you're using gcc you can do this by including -llwip in your link command.
+
+* Run your application
+
+ Ensure that LD_LIBRARY_PATH includes the directory that contains liblwip.so
+ (ie. this directory)
+
+
+If you are unsure about shared libraries and libraries on linux in
+general, you might find this HOWTO useful:
+
+<http://www.tldp.org/HOWTO/Program-Library-HOWTO/>