New code from lwip project

lwip repo:
678dcc2 Moved IPv4/IPv6 headers to common include folder, now that their names are unique -> no need to add 3 directories to the include path when compiling lwIP

lwip-contrib repo:
cf40c63 Adapted example ports to moved IPv4/IPv6 header files
878b4db fixed a 'use after free' found from scanning the our httpd with a webserver scanning tool