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/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