lwip: Support for linux target

Implement linux port layer and reuse the original FreeRTOS layer
that's compiled and used on linux target as well, by means of FreeRTOS
simulator.

components/pthread/CMakeLists.txt

@@ -1,3 +1,13 @@
+idf_build_get_property(target IDF_TARGET)
+if(${target} STREQUAL "linux")
+    # Make pthread component an empty interface lib referencing host pthread for Linux target
+    idf_component_register()
+    set(THREADS_PREFER_PTHREAD_FLAG ON)
+    find_package(Threads REQUIRED)
+    target_link_libraries(${COMPONENT_LIB} INTERFACE Threads::Threads)
+    return()
+endif()
+
 set(sources "pthread.c"
             "pthread_cond_var.c"
             "pthread_local_storage.c"