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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,23 @@
+#ifndef RECIPROCALLENGTH_HPP_6VHZDL7G
+#define RECIPROCALLENGTH_HPP_6VHZDL7G
+
+#include <boost/units/derived_dimension.hpp>
+#include <boost/units/physical_dimensions/length.hpp>
+#include <boost/units/systems/si/base.hpp>
+#include <boost/units/quantity.hpp>
+
+namespace vanetza
+{
+namespace units
+{
+
+typedef boost::units::derived_dimension<boost::units::length_base_dimension, -1>::type curvature_dimension;
+typedef boost::units::unit<curvature_dimension, boost::units::si::system> curvature;
+typedef boost::units::quantity<curvature> Curvature;
+
+BOOST_UNITS_STATIC_CONSTANT(reciprocal_metre, curvature);
+
+} // namespace units
+} // namespace vanetza
+
+#endif /* RECIPROCALLENGTH_HPP_6VHZDL7G */