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/v3/hash.cpp

@@ -0,0 +1,48 @@
+#include <vanetza/security/backend.hpp>
+#include <vanetza/security/v3/certificate.hpp>
+#include <vanetza/security/v3/hash.hpp>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v3
+{
+
+ByteBuffer calculate_message_hash(Backend& backend, HashAlgorithm hash_algo, const ByteBuffer& payload, const CertificateView& signing_cert)
+{
+    ByteBuffer encoded_cert;
+    if (signing_cert.is_canonical()) {
+        encoded_cert = signing_cert.encode();
+    } else {
+        auto canonical_signing_cert = signing_cert.canonicalize();
+        if (canonical_signing_cert) {
+            encoded_cert = canonical_signing_cert->encode();
+        }
+    }
+
+    ByteBuffer data_hash = backend.calculate_hash(hash_algo, payload);
+    ByteBuffer cert_hash = backend.calculate_hash(hash_algo, encoded_cert);
+    ByteBuffer concat_hash;
+    concat_hash.reserve(data_hash.size() + cert_hash.size());
+    concat_hash.insert(concat_hash.end(), data_hash.begin(), data_hash.end());
+    concat_hash.insert(concat_hash.end(), cert_hash.begin(), cert_hash.end());
+    return backend.calculate_hash(hash_algo, concat_hash);
+}
+
+HashAlgorithm specified_hash_algorithm(KeyType key_type)
+{
+    switch (key_type) {
+        case KeyType::NistP256:
+        case KeyType::BrainpoolP256r1:
+            return HashAlgorithm::SHA256;
+        case KeyType::BrainpoolP384r1:
+            return HashAlgorithm::SHA384;
+        default:
+            return HashAlgorithm::Unspecified;
+    }
+}
+
+} // namespace v3
+} // namespace security
+} // namespace vanetza