Replace symlink with actual file. (#2856)

Due to Windows limitations. Sigh.

Author: jmattsson

sdk-overrides/include/espconn.h

@@ -0,0 +1,5 @@
+#ifndef _SDK_OVERRIDE_ESPCONN_H_
+#define _SDK_OVERRIDE_ESPCONN_H_
+// Pull in the correct lwIP header
+#include "../../app/include/lwip/app/espconn.h"
+#endif