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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,34 @@
+#include <vanetza/common/clock.hpp>
+#include <boost/date_time/posix_time/posix_time.hpp>
+
+namespace vanetza
+{
+
+const boost::posix_time::ptime& Clock::epoch()
+{
+    static const boost::posix_time::ptime epoch {
+        boost::gregorian::date(2004, 1, 1),
+        boost::posix_time::milliseconds(0)
+    };
+    return epoch;
+}
+
+Clock::time_point Clock::at(const boost::posix_time::ptime& t)
+{
+    auto delta = (t - epoch()).total_microseconds();
+    Clock::time_point tp { std::chrono::microseconds(delta) };
+    return tp;
+}
+
+Clock::date_time Clock::at(const Clock::time_point& t)
+{
+    std::chrono::microseconds delta = t.time_since_epoch();
+    return Clock::epoch() + boost::posix_time::microseconds(delta.count());
+}
+
+Clock::time_point Clock::at(const std::string& at)
+{
+    return Clock::at(boost::posix_time::time_from_string(at));
+}
+
+} // namespace vanetza