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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,27 @@
+#ifndef BEACON_HEADER_HPP_1NRWPHXO
+#define BEACON_HEADER_HPP_1NRWPHXO
+
+#include <vanetza/geonet/position_vector.hpp>
+#include <vanetza/geonet/serialization.hpp>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+struct BeaconHeader
+{
+public:
+    static constexpr std::size_t length_bytes = LongPositionVector::length_bytes;
+
+    LongPositionVector source_position;
+};
+
+void serialize(const BeaconHeader&, OutputArchive&);
+void deserialize(BeaconHeader&, InputArchive&);
+
+} // namespace geonet
+} // namespace vanetza
+
+#endif /* BEACON_HEADER_HPP_1NRWPHXO */
+