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/vanetza/security/backend.cpp

@@ -0,0 +1,74 @@
+#include <vanetza/common/factory.hpp>
+#include <vanetza/security/backend.hpp>
+#include <cassert>
+#include <stdexcept>
+
+namespace
+{
+
+static vanetza::Factory<vanetza::security::Backend> backend_factory;
+
+template<typename T>
+class BackendRegistrar
+{
+public:
+    BackendRegistrar(const std::string& name)
+    {
+        auto f = []() { return std::unique_ptr<vanetza::security::Backend> { new T() }; };
+        bool success = backend_factory.add(name, f) && backend_factory.configure_default(name);
+        assert(success);
+    }
+};
+
+} // namespace
+
+// order of VANETZA_REGISTER_CRYPTO_BACKEND invocation defines priority: last registered one will be default
+#define VANETZA_REGISTER_CRYPTO_BACKEND(clazz) \
+    namespace { \
+        using namespace vanetza::security; \
+        static const BackendRegistrar<clazz> clazz##Factory(clazz::backend_name); \
+    }
+
+#include <vanetza/security/backend_null.hpp>
+VANETZA_REGISTER_CRYPTO_BACKEND(BackendNull)
+
+#ifdef VANETZA_WITH_CRYPTOPP
+#   include <vanetza/security/backend_cryptopp.hpp>
+    VANETZA_REGISTER_CRYPTO_BACKEND(BackendCryptoPP)
+#endif
+
+#ifdef VANETZA_WITH_OPENSSL
+#   include <vanetza/security/backend_openssl.hpp>
+    VANETZA_REGISTER_CRYPTO_BACKEND(BackendOpenSsl)
+#endif
+
+namespace vanetza
+{
+namespace security
+{
+
+const Factory<Backend>& builtin_backends()
+{
+    return backend_factory;
+}
+
+std::unique_ptr<Backend> create_backend(const std::string& name, const Factory<Backend>& factory)
+{
+    if (name == "default") {
+        return factory.create();
+    } else {
+        return factory.create(name);
+    }
+}
+
+std::unique_ptr<Backend> create_backend_or_throw(const std::string& name)
+{
+    auto backend = create_backend(name);
+    if (!backend) {
+        throw std::runtime_error("security backend unavailable");
+    }
+    return backend;
+}
+
+} // namespace security
+} // namespace vanetza