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FLASHING.md says how to take a newer version of the colleague's tree (copy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/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 */
+