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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,56 @@
+#ifndef LINKLAYER_ENDPOINT_HPP_DMNJI8U6
+#define LINKLAYER_ENDPOINT_HPP_DMNJI8U6
+
+#include <boost/asio/detail/socket_ops.hpp>
+#include <boost/asio/detail/socket_types.hpp>
+#include <netpacket/packet.h>
+
+namespace vanetza
+{
+namespace asio
+{
+
+template<typename Protocol>
+class linklayer_endpoint
+{
+    public:
+    typedef boost::asio::detail::socket_addr_type sockaddr_type;
+    typedef sockaddr_ll sockaddr_ll_type;
+
+    linklayer_endpoint() : m_data() {
+        Protocol protocol;
+        m_data.link.sll_family = protocol.family();
+        m_data.link.sll_protocol = boost::asio::detail::socket_ops::host_to_network_short(protocol.protocol());
+        m_data.link.sll_ifindex = protocol.interface_index();
+    }
+
+    linklayer_endpoint(const Protocol& protocol) : m_data() {
+        m_data.link.sll_family = protocol.family();
+        m_data.link.sll_protocol = boost::asio::detail::socket_ops::host_to_network_short(protocol.protocol());
+        m_data.link.sll_ifindex = protocol.interface_index();
+    }
+
+    const sockaddr_type* data() const
+    {
+        return &m_data.base;
+    }
+
+    std::size_t size() const
+    {
+        return sizeof(sockaddr_ll_type);
+    }
+
+    Protocol protocol() const { return Protocol(); }
+
+    private:
+    union data_union {
+        sockaddr_type base;
+        sockaddr_ll_type link;
+    } m_data;
+};
+
+} // namespace asio
+} // namespace vanetza
+
+#endif /* LINKLAYER_ENDPOINT_HPP_DMNJI8U6 */
+