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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,30 @@
+#ifndef SHB_HEADER_HPP_MRLDRPNK
+#define SHB_HEADER_HPP_MRLDRPNK
+
+#include <vanetza/geonet/dcc_field.hpp>
+#include <vanetza/geonet/position_vector.hpp>
+#include <cstdint>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+struct ShbHeader
+{
+    ShbHeader();
+
+    static constexpr std::size_t length_bytes = 4 + LongPositionVector::length_bytes;
+
+    LongPositionVector source_position;
+    DccField dcc; /* < this field is "reserved" in 102 636-4-1 */
+};
+
+void serialize(const ShbHeader&, OutputArchive&);
+void deserialize(ShbHeader&, InputArchive&);
+
+} // namespace geonet
+} // namespace vanetza
+
+#endif /* SHB_HEADER_HPP_MRLDRPNK */
+