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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,73 @@
+#include "ethernet_device.hpp"
+#include <boost/asio/ip/address.hpp>
+#include <algorithm>
+#include <cstring>
+#include <system_error>
+#include <linux/if_ether.h>
+#include <linux/if_packet.h>
+#include <net/if.h>
+#include <sys/ioctl.h>
+#include <ifaddrs.h>
+
+static void initialize(ifreq& request, const char* interface_name)
+{
+    std::memset(&request, 0, sizeof(ifreq));
+    std::strncpy(request.ifr_name, interface_name, IF_NAMESIZE);
+    request.ifr_name[IF_NAMESIZE - 1] = '\0';
+}
+
+EthernetDevice::EthernetDevice(const char* devname) :
+    inet_socket_(::socket(AF_INET, SOCK_DGRAM, 0)),
+    interface_name_(devname)
+{
+    if (!inet_socket_) {
+        throw std::system_error(errno, std::system_category());
+    }
+}
+
+EthernetDevice::~EthernetDevice()
+{
+    if (inet_socket_ >= 0)
+        ::close(inet_socket_);
+
+}
+
+EthernetDevice::protocol::endpoint EthernetDevice::endpoint(int family) const
+{
+    sockaddr_ll socket_address = {};
+    socket_address.sll_family = family;
+    socket_address.sll_protocol = htons(ETH_P_ALL);
+    socket_address.sll_ifindex = index();
+    return protocol::endpoint(&socket_address, sizeof(sockaddr_ll));
+}
+
+int EthernetDevice::index() const
+{
+    ifreq data;
+    initialize(data, interface_name_.c_str());
+    ::ioctl(inet_socket_, SIOCGIFINDEX, &data);
+    return data.ifr_ifindex;
+}
+
+vanetza::MacAddress EthernetDevice::address() const
+{
+    ifreq data;
+    initialize(data, interface_name_.c_str());
+    ::ioctl(inet_socket_, SIOCGIFHWADDR, &data);
+
+    vanetza::MacAddress addr;
+    std::copy_n(data.ifr_hwaddr.sa_data, addr.octets.size(), addr.octets.data());
+    return addr;
+}
+
+boost::asio::ip::address_v4 EthernetDevice::ip() const
+{
+    ifreq data;
+    initialize(data, interface_name_.c_str());
+    ::ioctl(inet_socket_, SIOCGIFADDR, &data);
+
+    char host[NI_MAXHOST] = { 0 };
+    ::getnameinfo(&data.ifr_addr, sizeof(sockaddr), host, NI_MAXHOST, nullptr, 0, NI_NUMERICHOST);
+
+    return boost::asio::ip::make_address_v4(host);
+}