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FLASHING.md says how to take a newer version of the colleague's tree (copy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/link_layer.hpp

@@ -0,0 +1,38 @@
+#ifndef LINK_LAYER_HPP_FGEK0QTH
+#define LINK_LAYER_HPP_FGEK0QTH
+
+#include "ethernet_device.hpp"
+#include <vanetza/access/interface.hpp>
+#include <vanetza/net/cohesive_packet.hpp>
+#include <vanetza/net/ethernet_header.hpp>
+#include <boost/asio/io_context.hpp>
+#include <boost/asio/ip/address.hpp>
+#include <boost/program_options/options_description.hpp>
+#include <boost/program_options/variables_map.hpp>
+#include <memory>
+#include <string>
+
+class LinkLayerIndication
+{
+public:
+    using IndicationCallback = std::function<void(vanetza::CohesivePacket&&, const vanetza::EthernetHeader&)>;
+
+    virtual void indicate(IndicationCallback) = 0;
+    virtual ~LinkLayerIndication() = default;
+};
+
+class LinkLayer : public vanetza::access::Interface, public LinkLayerIndication
+{
+public:
+    virtual void set_source_address(const vanetza::MacAddress&) {};
+};
+
+boost::optional<std::pair<boost::asio::ip::address, unsigned short>> parse_ip_port(const std::string& ip_port);
+
+std::unique_ptr<LinkLayer>
+create_link_layer(boost::asio::io_context&, const EthernetDevice&, const std::string& name, const boost::program_options::variables_map& vm);
+
+void add_link_layer_options(boost::program_options::options_description&);
+
+#endif /* LINK_LAYER_HPP_FGEK0QTH */
+