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FLASHING.md says how to take a newer version of the colleague's tree (copy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/certify/commands/generate-key.cpp

@@ -0,0 +1,56 @@
+#include "generate-key.hpp"
+#include <boost/program_options.hpp>
+#include <fstream>
+#include <iostream>
+#include <vanetza/security/backend.hpp>
+#include <vanetza/security/v2/persistence.hpp>
+
+namespace po = boost::program_options;
+using namespace vanetza::security;
+
+bool GenerateKeyCommand::parse(const std::vector<std::string>& opts)
+{
+    po::options_description desc("Available options");
+    desc.add_options()
+        ("help", "Print out available options.")
+        ("output", po::value<std::string>(&output)->required(), "Output file.")
+    ;
+
+    po::positional_options_description pos;
+    pos.add("output", 1);
+
+    po::variables_map vm;
+    po::store(po::command_line_parser(opts).options(desc).positional(pos).run(), vm);
+
+    if (vm.count("help")) {
+        std::cerr << desc << std::endl;
+
+        return false;
+    }
+
+    try {
+        po::notify(vm);
+    } catch (const std::exception& e) {
+        std::cerr << "Error: " << e.what() << std::endl << std::endl << desc << std::endl;
+
+        return false;
+    }
+
+    return true;
+}
+
+int GenerateKeyCommand::execute()
+{
+    auto backend = create_backend_or_throw("default");
+
+    std::cout << "Generating key... ";
+    auto key_pair = backend->generate_key_pair();
+    std::cout << "OK" << std::endl;
+
+    std::cout << "Writing key to '" << output << "'... ";
+    std::ofstream ofs(output, std::ios::binary);
+    v2::save_private_key_pkcs8_der(ofs, key_pair);
+    std::cout << "OK" << std::endl;
+
+    return 0;
+}