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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,54 @@
+#ifndef COMMON_HEADER_HPP_SEFIWCT4
+#define COMMON_HEADER_HPP_SEFIWCT4
+
+#include <vanetza/common/bit_number.hpp>
+#include <vanetza/geonet/header_type.hpp>
+#include <vanetza/geonet/mib.hpp>
+#include <vanetza/geonet/position_vector.hpp>
+#include <vanetza/geonet/serialization.hpp>
+#include <vanetza/geonet/traffic_class.hpp>
+#include <cstdint>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+struct Area;
+struct DataRequest;
+struct ShbDataRequest;
+class LocationTable;
+
+enum class NextHeaderCommon
+{
+    Any = 0, BTP_A = 1, BTP_B = 2, IPv6 = 3
+};
+
+struct CommonHeader
+{
+public:
+    CommonHeader();
+    CommonHeader(const MIB&);
+    CommonHeader(const DataRequest&, const MIB&);
+    CommonHeader(const ShbDataRequest&, const MIB&);
+
+    static constexpr std::size_t length_bytes = 8;
+
+    NextHeaderCommon next_header; // 4 bit
+    BitNumber<unsigned, 4> reserved1;
+    HeaderType header_type;
+    TrafficClass traffic_class;
+    uint8_t flags; // Bit 0: itsGnIsMobile
+    uint16_t payload; // number of octets following whole GeoNet header
+    uint8_t maximum_hop_limit;
+    uint8_t reserved2;
+};
+
+void serialize(const CommonHeader&, OutputArchive&);
+void deserialize(CommonHeader&, InputArchive&);
+
+} // namespace geonet
+} // namespace vanetza
+
+#endif /* COMMON_HEADER_HPP_SEFIWCT4 */
+