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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,61 @@
+#include "xdg.hpp"
+#include <cstdlib>
+#include <pwd.h>
+#include <stdexcept>
+#include <unistd.h>
+
+namespace vanetza
+{
+namespace pki
+{
+
+namespace
+{
+
+std::filesystem::path lookup_xdg_env(const char* env)
+{
+    const char* env_value = std::getenv(env);
+    if (env_value) {
+        return std::filesystem::path(env_value);
+    } else {
+        return std::filesystem::path {};
+    }
+}
+
+std::filesystem::path lookup_home()
+{
+    const char* home_value = std::getenv("HOME");
+    if (home_value && home_value[0] != '\0') {
+        return std::filesystem::path { home_value };
+    }
+
+    const passwd* pw = ::getpwuid(getuid());
+    if (pw && pw->pw_dir) {
+        return std::filesystem::path { pw->pw_dir };
+    }
+
+    throw std::runtime_error("cannot determine home directory");
+}
+
+} // namespace
+
+std::filesystem::path xdg_config_home()
+{
+    auto config_home = lookup_xdg_env("XDG_CONFIG_HOME");
+    if (config_home.empty()) {
+        config_home = lookup_home() / ".config";
+    }
+    return config_home;
+}
+
+std::filesystem::path xdg_data_home()
+{
+    auto data_home = lookup_xdg_env("XDG_DATA_HOME");
+    if (data_home.empty()) {
+        data_home = lookup_home() / ".local/share";
+    }
+    return data_home;
+}
+
+} // namespace pki
+} // namespace vanetza