Keep vanetza-idf in obu-firmware, so a plain clone builds the firmware

obu-firmware builds against the vanetza-idf C-ITS library, which until now
came from the colleague's microbu-esp32c5 tree beside the repository and was
not tracked here, so a clone of this repository could not build the firmware
it ships. The library alone is now part of obu-firmware, as
obu-firmware/external/vanetza-idf: their external/vanetza-idf at commit
cf4b99f, unchanged (9775 files; see its PROVENANCE.md). CMake takes it from
there by default; -DVANETZA_IDF_DIR still points the build elsewhere.

The rest of the colleague's tree (their own VAM firmware, PKI tooling,
station-link Python tools, the V2X2MAP bridge) stays out of this repository
and gitignored; nothing is pushed to their repository. NOTES.md, docs/06,
TODO.md and the pcap verifier's usage line point at the new location.

obu-firmware/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 */
+