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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,44 @@
+#pragma once
+
+#include "security_module.hpp"
+#include <boost/optional/optional.hpp>
+
+namespace vanetza
+{
+namespace pki
+{
+
+class CredentialStorage;
+
+class OpenSslSecurityModule : public SecurityModule
+{
+public:
+    OpenSslSecurityModule(std::shared_ptr<CredentialStorage>);
+
+    Sha256Hash calculate_sha256_hash(const std::uint8_t* buffer, std::size_t length) override;
+    Sha384Hash calculate_sha384_hash(const std::uint8_t* buffer, std::size_t length) override;
+    bool verify(const Sha256Hash&, const Signature&, const PublicKey&) override;
+    bool verify(const Sha384Hash&, const Signature&, const PublicKey&) override;
+    ByteBuffer generate_nonce(std::size_t length) override;
+    std::unique_ptr<EciesContext> create_ecies_context(const PublicKey& receiver, const Sha256Hash& info) override;
+    ByteBuffer calculate_hmac_sha256(const ByteBuffer& key, const ByteBuffer& data) override;
+
+    boost::optional<Signature> sign(const ByteBuffer& data, const PublicKey&) override;
+
+    /**
+     * Creates a new public and private key pair.
+     * The private key is intentionally not returned but only stored in the credential storage.
+     */
+    PublicKey create_key(KeyType) override;
+
+    bool discard_key(const PublicKey&) override;
+
+private:
+    bool verify(const uint8_t* digest, std::size_t dlen, const Signature&, const PublicKey&);
+    boost::optional<Signature> sign(const ByteBuffer&, const PrivateKey&);
+
+    std::shared_ptr<CredentialStorage> m_credential_storage;
+};
+
+} // namespace pki
+} // namespace vanetza