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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,76 @@
+#include "crl_store.hpp"
+#include "certificate_revocation_list.hpp"
+#include "filesystem.hpp"
+#include "hexstring.hpp"
+#include "security_module.hpp"
+
+namespace vanetza
+{
+namespace pki
+{
+
+CrlFilesystemStore::CrlFilesystemStore(std::shared_ptr<SecurityModule> security, const std::filesystem::path& root) :
+    m_security(security), m_root(root)
+{
+    std::filesystem::create_directories(m_root);
+    for (const auto& entry : std::filesystem::directory_iterator(m_root)) {
+        if (!entry.is_regular_file() || entry.path().extension() != ".crl") {
+            continue;
+        }
+        ByteBuffer buf = read(entry.path());
+        CertificateRevocationList crl;
+        if (crl.decode(buf)) {
+            index(crl);
+        }
+    }
+}
+
+bool CrlFilesystemStore::store(const CertificateRevocationList& crl)
+{
+    auto issuer = index(crl);
+    if (!issuer) {
+        return false;
+    }
+    write(filename(*issuer), crl.encode());
+    return true;
+}
+
+bool CrlFilesystemStore::is_revoked(const security::HashedId8& issuer, const security::HashedId8& cert) const
+{
+    auto it = m_revoked.find(issuer);
+    if (it == m_revoked.end()) {
+        return false;
+    }
+    return it->second.count(cert) != 0;
+}
+
+std::filesystem::path CrlFilesystemStore::filename(const HashedId8& id) const
+{
+    return m_root / (hexstring(id.octets) + ".crl");
+}
+
+boost::optional<HashedId8> CrlFilesystemStore::index(const CertificateRevocationList& crl)
+{
+    auto issuer = crl.get_hashed_id8(*m_security);
+    if (!issuer) {
+        return boost::none;
+    }
+    auto entries = crl.revoked_entries();
+    if (!entries) {
+        return boost::none;
+    }
+
+    // Empty CRL is valid: it withdraws any prior revocations from this issuer.
+    // Erase the existing bucket and only create a new one if there's content.
+    m_revoked.erase(issuer->octets);
+    if (!entries->empty()) {
+        auto& bucket = m_revoked[issuer->octets];
+        for (const auto& cert : *entries) {
+            bucket.insert(cert.octets);
+        }
+    }
+    return issuer;
+}
+
+} // namespace pki
+} // namespace vanetza