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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,78 @@
+#include <vanetza/common/byte_sequence.hpp>
+#include <vanetza/security/backend_null.hpp>
+#include <vanetza/security/public_key.hpp>
+#include <vanetza/security/signature.hpp>
+
+namespace vanetza
+{
+namespace security
+{
+
+EcdsaSignature BackendNull::sign_data(const ecdsa256::PrivateKey&, const ByteBuffer&)
+{
+    static const EcdsaSignature fake = fake_signature();
+    return fake;
+}
+
+Signature BackendNull::sign_digest(const PrivateKey&, const ByteBuffer&)
+{
+    static const Signature empty {};
+    return empty;
+}
+
+bool BackendNull::verify_data(const ecdsa256::PublicKey&, const ByteBuffer&, const EcdsaSignature&)
+{
+    // accept everything
+    return true;
+}
+
+bool BackendNull::verify_digest(const PublicKey&, const ByteBuffer&, const Signature&)
+{
+    // accept everything
+    return true;
+}
+
+boost::optional<Uncompressed> BackendNull::decompress_point(const EccPoint&)
+{
+    return boost::none;
+}
+
+EcdsaSignature BackendNull::fake_signature() const
+{
+    constexpr std::size_t size = 32;
+    EcdsaSignature signature;
+    X_Coordinate_Only coordinate;
+    coordinate.x = random_byte_sequence(size, 0xdead);
+    signature.R = coordinate;
+    signature.s = random_byte_sequence(size, 0xbeef);
+
+    return signature;
+}
+
+ByteBuffer BackendNull::calculate_hash(HashAlgorithm algo, const ByteBuffer&)
+{
+    ByteBuffer hash;
+    switch (algo) {
+        case HashAlgorithm::SHA256:
+            hash.resize(32);
+            break;
+        case HashAlgorithm::SHA384:
+            hash.resize(48);
+            break;
+        default:
+            break;
+    }
+    return hash;
+}
+
+ecdsa256::KeyPair BackendNull::generate_key_pair()
+{
+    ecdsa256::KeyPair key_pair;
+    key_pair.private_key.key.fill(0);
+    key_pair.public_key.x.fill(0);
+    key_pair.public_key.y.fill(0);
+    return key_pair;
+}
+
+} // namespace security
+} // namespace vanetza