Keep vanetza-idf in obu-firmware, so a plain clone builds the firmware

obu-firmware builds against the vanetza-idf C-ITS library, which until now
came from the colleague's microbu-esp32c5 tree beside the repository and was
not tracked here, so a clone of this repository could not build the firmware
it ships. The library alone is now part of obu-firmware, as
obu-firmware/external/vanetza-idf: their external/vanetza-idf at commit
cf4b99f, unchanged (9775 files; see its PROVENANCE.md). CMake takes it from
there by default; -DVANETZA_IDF_DIR still points the build elsewhere.

The rest of the colleague's tree (their own VAM firmware, PKI tooling,
station-link Python tools, the V2X2MAP bridge) stays out of this repository
and gitignored; nothing is pushed to their repository. NOTES.md, docs/06,
TODO.md and the pcap verifier's usage line point at the new location.

obu-firmware/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