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.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