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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,42 @@
+#include "gbc_header.hpp"
+#include "areas.hpp"
+#include <stdexcept>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+Area GeoBroadcastHeader::destination(HeaderType ht) const
+{
+    decltype(Area::shape) shape;
+    switch (ht) {
+        case HeaderType::GeoBroadcast_Circle:
+            shape = Circle();
+            break;
+        case HeaderType::GeoBroadcast_Rect:
+            shape = Rectangle();
+            break;
+        case HeaderType::GeoBroadcast_Elip:
+            shape = Ellipse();
+            break;
+        default:
+            throw std::runtime_error("Invalid GBC header type");
+            break;
+    }
+
+    return detail::GbcGacHeader::destination(shape);
+}
+
+void serialize(const GeoBroadcastHeader& header, OutputArchive& ar)
+{
+    detail::serialize(header, ar);
+}
+
+void deserialize(GeoBroadcastHeader& header, InputArchive& ar)
+{
+    detail::deserialize(header, ar);
+}
+
+} // namespace geonet
+} // namespace vanetza