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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,40 @@
+#ifndef CLOCK_HPP_2FCBLXSJ
+#define CLOCK_HPP_2FCBLXSJ
+
+#include <boost/date_time/posix_time/posix_time_types.hpp>
+#include <chrono>
+#include <cstdint>
+#include <ratio>
+
+namespace vanetza
+{
+
+/**
+ * A Clock similar to std::chrono with epoch at 2004-01-01 at midnight
+ */
+class Clock
+{
+public:
+    typedef int64_t rep;
+    typedef std::ratio<1, 1000 * 1000> period;
+    typedef std::chrono::duration<rep, period> duration;
+    typedef std::chrono::time_point<Clock> time_point;
+    typedef boost::posix_time::ptime date_time;
+
+    static constexpr bool is_steady() { return true; }
+    static time_point at(const date_time&);
+    static date_time at(const time_point&);
+    static const date_time& epoch();
+
+    /**
+     * \brief create time point
+     * \param at time string formatted like 2016-07-15 09:48:32
+     * \return time point
+     */
+    static time_point at(const std::string& at);
+};
+
+} // namespace vanetza
+
+#endif /* CLOCK_HPP_2FCBLXSJ */
+