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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,45 @@
+#pragma once
+
+#include "hashed_id8.hpp"
+#include <vanetza/security/hashed_id.hpp>
+#include <vanetza/security/v3/revocation_lookup.hpp>
+#include <boost/optional/optional.hpp>
+#include <filesystem>
+#include <memory>
+#include <unordered_map>
+#include <unordered_set>
+
+namespace vanetza
+{
+namespace pki
+{
+
+class CertificateRevocationList;
+class SecurityModule;
+
+/**
+ * Constructor scans \p root and indexes every "*.crl" file.
+ * store() replaces any prior CRL from the same issuer (one file per issuer).
+ */
+class CrlFilesystemStore : public security::v3::RevocationLookup
+{
+public:
+    CrlFilesystemStore(std::shared_ptr<SecurityModule>, const std::filesystem::path& root);
+
+    // Returns false if the CRL has no extractable issuer or a malformed payload.
+    bool store(const CertificateRevocationList&);
+
+    bool is_revoked(const security::HashedId8& issuer, const security::HashedId8& cert) const override;
+
+protected:
+    std::filesystem::path filename(const HashedId8&) const;
+    boost::optional<HashedId8> index(const CertificateRevocationList&);
+
+private:
+    std::shared_ptr<SecurityModule> m_security;
+    std::filesystem::path m_root;
+    std::unordered_map<security::HashedId8, std::unordered_set<security::HashedId8>> m_revoked;
+};
+
+} // namespace pki
+} // namespace vanetza