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FLASHING.md says how to take a newer version of the colleague's tree (copy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