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/vanetza/geonet/dcc_field.hpp

@@ -0,0 +1,32 @@
+#ifndef DCC_FIELD_HPP_EPTBYHAU
+#define DCC_FIELD_HPP_EPTBYHAU
+
+#include <vanetza/geonet/dcc_mco_field.hpp>
+#include <vanetza/geonet/serialization.hpp>
+#include <boost/optional/optional.hpp>
+#include <boost/variant/variant.hpp>
+#include <cstdint>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+/**
+ * DccField represents the supported variants for the DCC field in SHB headers.
+ *
+ * \note Enclosed types are using "host byte order".
+ *       Byte order conversion is handled by serialize and deserialize functions.
+ */
+using DccField = boost::variant<DccMcoField, uint32_t>;
+
+boost::optional<DccMcoField> get_dcc_mco(const DccField&);
+
+void serialize(const DccField&, OutputArchive&);
+void deserialize(DccField&, InputArchive&);
+
+} // namespace geonet
+} // namespace vanetza
+
+#endif /* DCC_FIELD_HPP_EPTBYHAU */
+