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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,56 @@
+#include <vanetza/geonet/dcc_field.hpp>
+#include <boost/variant/apply_visitor.hpp>
+#include <boost/variant/static_visitor.hpp>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+struct dcc_mco_extractor : boost::static_visitor< boost::optional<DccMcoField> >
+{
+    using return_type = boost::optional<DccMcoField>;
+
+    return_type operator()(const DccMcoField& mco) const
+    {
+        return mco;
+    }
+
+    return_type operator()(uint32_t raw) const
+    {
+        return DccMcoField { raw };
+    }
+
+    template<typename T>
+    return_type operator()(const T&)
+    {
+        return boost::none;
+    }
+};
+
+boost::optional<DccMcoField> get_dcc_mco(const DccField& field)
+{
+    return boost::apply_visitor(dcc_mco_extractor(), field);
+}
+
+void serialize(const DccField& field, OutputArchive& ar)
+{
+    struct serialize_visitor : boost::static_visitor<uint32_t>
+    {
+        uint32_t operator()(const DccMcoField& mco) const { return static_cast<uint32_t>(mco); }
+        uint32_t operator()(const uint32_t& raw) const { return raw; }
+    };
+
+    uint32_t raw = boost::apply_visitor(serialize_visitor(), field);
+    serialize(host_cast(raw), ar);
+}
+
+void deserialize(DccField& field, InputArchive& ar)
+{
+    uint32_t raw;
+    deserialize(raw, ar);
+    field = raw;
+}
+
+} // namespace geonet
+} // namespace vanetza