Keep the colleague's microbu-esp32c5 tree in this repository

obu-firmware builds against vanetza-idf from microbu-esp32c5/external, but
that tree was gitignored, so a clone of this repository could not build the
firmware it ships. It is now committed here as ordinary files in its own
folder, microbu-esp32c5/: the colleague's commit cf4b99f plus the V2X2MAP
bridge's signature verification (--trust) used on the bench. Nothing is
fetched from or pushed to the colleague's repository; this repository and
its remotes carry everything. The folder's own .gitignore keeps build output,
downloaded components and private key material out, as it did there; the
committed file set is identical to that repository's tracked files.

The ESP32-C5 is still flashed from obu-firmware/, which only takes
vanetza-idf from microbu-esp32c5/, so the two stay separate folders.
FLASHING.md says how to take a newer version of the colleague's tree (copy
it over the folder, rebuild, test, commit).

microbu-esp32c5/external/vanetza-idf/tools/socktap/tcp_link.hpp

@@ -0,0 +1,75 @@
+#ifndef TCP_LINK_HPP_A16QFBX3
+#define TCP_LINK_HPP_A16QFBX3
+
+#include "link_layer.hpp"
+#include <vanetza/common/byte_buffer.hpp>
+#include <boost/asio/io_context.hpp>
+#include <boost/asio/ip/tcp.hpp>
+#include <array>
+#include <list>
+
+static constexpr std::size_t layers_ = num_osi_layers(vanetza::OsiLayer::Physical, vanetza::OsiLayer::Application);
+
+
+class TcpSocket
+{
+public:
+    enum Status
+    {
+        UNDEFINED = 0,
+        CONNECTED = 1,
+        ACCEPTING = 2,
+        ERROR = 3
+    };
+
+    using IndicationCallback = std::function<void(vanetza::CohesivePacket&&, const vanetza::EthernetHeader&)>;
+
+    TcpSocket(boost::asio::io_context&, IndicationCallback*);
+
+    void connect(boost::asio::ip::tcp::endpoint);
+    void request(std::array<boost::asio::const_buffer, layers_>);
+    void do_receive();
+    void receive_handler(boost::system::error_code, std::size_t);
+    std::size_t get_next_packet_size();
+    void pass_up(vanetza::ByteBuffer&&);
+
+    boost::asio::ip::tcp::socket& socket() { return socket_; }
+
+    Status status() { return status_; }
+    void status(Status s) { status_ = s; }
+
+private:
+    Status status_ = UNDEFINED;
+    boost::asio::io_context* io_context_;
+    boost::asio::ip::tcp::endpoint endpoint_;
+    boost::asio::ip::tcp::socket socket_;
+    vanetza::ByteBuffer rx_buffer_;
+    vanetza::ByteBuffer rx_store_;
+    IndicationCallback* callback_;
+
+};
+
+
+class TcpLink : public LinkLayer
+{
+public:
+    TcpLink(boost::asio::io_context&);
+
+    void indicate(IndicationCallback) override;
+    void request(const vanetza::access::DataRequest&, std::unique_ptr<vanetza::ChunkPacket>) override;
+    void connect(boost::asio::ip::tcp::endpoint);
+    void accept(boost::asio::ip::tcp::endpoint);
+    void accept_handler(boost::system::error_code& ec, boost::asio::ip::tcp::endpoint ep, TcpSocket* sock);
+
+private:
+    std::list<TcpSocket> sockets_;
+    std::map<boost::asio::ip::tcp::endpoint, boost::asio::ip::tcp::acceptor> acceptors_;
+    IndicationCallback callback_;
+    boost::asio::io_context* io_context_;
+    std::array<vanetza::ByteBuffer, layers_> tx_buffers_;
+    std::list<boost::asio::ip::tcp::endpoint> waiting_endpoints_;
+};
+
+
+#endif /* TCP_LINK_HPP_A16QFBX3 */
+