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_null.hpp

@@ -0,0 +1,51 @@
+#ifndef BACKEND_NULL_HPP_3E6GMTPL
+#define BACKEND_NULL_HPP_3E6GMTPL
+
+#include <vanetza/security/backend.hpp>
+
+namespace vanetza
+{
+namespace security
+{
+
+/**
+ * \brief A backend doing nothing
+ *
+ * This backend is INSECURE quite obviously!
+ * It will return a faked signature and silently accept all data at verification.
+ */
+class BackendNull : public Backend
+{
+public:
+    static constexpr auto backend_name = "Null";
+
+    /// \see Backend::sign_data
+    EcdsaSignature sign_data(const ecdsa256::PrivateKey& private_key, const ByteBuffer& data_buffer) override;
+
+    /// \see Backend::sign_digest
+    Signature sign_digest(const PrivateKey&, const ByteBuffer&) override;
+
+    /// \see Backend::verify_data
+    bool verify_data(const ecdsa256::PublicKey& public_key, const ByteBuffer& data, const EcdsaSignature& sig) override;
+
+    /// \see Backend::verify_digest
+    bool verify_digest(const PublicKey&, const ByteBuffer& digest, const Signature&) override;
+
+    /// \see Backend::decompress_point
+    boost::optional<Uncompressed> decompress_point(const EccPoint& ecc_point) override;
+
+    /// \see Backend::calculate_hash
+    ByteBuffer calculate_hash(HashAlgorithm, const ByteBuffer&) override;
+
+    /// \see Backend::generate_key_pair
+    ecdsa256::KeyPair generate_key_pair() override;
+
+private:
+    EcdsaSignature fake_signature() const;
+};
+
+} // namespace security
+} // namespace vanetza
+
+#endif /* BACKEND_NULL_HPP_3E6GMTPL */
+