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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,47 @@
+#include "asn1.hpp"
+#include "certificate_trust_list.hpp"
+#include "hashed_id8.hpp"
+
+namespace vanetza
+{
+namespace pki
+{
+
+boost::optional<std::string> lookup_dc_url(const std::filesystem::path& ectl_file, const HashedId8& root_ca)
+{
+    struct DcUrlFinder : CtlVisitor
+    {
+        const HashedId8& root_ca;
+        boost::optional<std::string> url;
+
+        explicit DcUrlFinder(const HashedId8& ca) : root_ca(ca)
+        {
+        }
+
+        void add_distribution_centre(const Vanetza_Security_DcEntry_t& dc) override
+        {
+            if (url) {
+                return;
+            }
+            for (int j = 0; j < dc.cert.list.count; ++j) {
+                if (dc.cert.list.array[j] && equals(*dc.cert.list.array[j], root_ca)) {
+                    url = to_string(dc.url);
+                    return;
+                }
+            }
+        }
+    };
+
+    try {
+        CertificateTrustList ctl = CertificateTrustList::from_file(ectl_file);
+        DcUrlFinder finder { root_ca };
+        ctl.visit_tlm_ctl(finder);
+        return finder.url;
+    } catch (const std::exception&) {
+        // ECTL missing or malformed: treat as "no DC URL available"
+        return boost::none;
+    }
+}
+
+} // namespace pki
+} // namespace vanetza