Keep vanetza-idf in obu-firmware, so a plain clone builds the firmware

obu-firmware builds against the vanetza-idf C-ITS library, which until now
came from the colleague's microbu-esp32c5 tree beside the repository and was
not tracked here, so a clone of this repository could not build the firmware
it ships. The library alone is now part of obu-firmware, as
obu-firmware/external/vanetza-idf: their external/vanetza-idf at commit
cf4b99f, unchanged (9775 files; see its PROVENANCE.md). CMake takes it from
there by default; -DVANETZA_IDF_DIR still points the build elsewhere.

The rest of the colleague's tree (their own VAM firmware, PKI tooling,
station-link Python tools, the V2X2MAP bridge) stays out of this repository
and gitignored; nothing is pushed to their repository. NOTES.md, docs/06,
TODO.md and the pcap verifier's usage line point at the new location.

obu-firmware/external/vanetza-idf/vanetza/units/time.hpp

@@ -0,0 +1,51 @@
+#ifndef TIME_HPP_GFC1AX6E
+#define TIME_HPP_GFC1AX6E
+
+#include <vanetza/common/clock.hpp>
+#include <boost/units/quantity.hpp>
+#include <boost/units/systems/si/time.hpp>
+
+namespace vanetza
+{
+namespace units
+{
+
+namespace si = boost::units::si;
+typedef boost::units::quantity<boost::units::si::time> Duration;
+
+/**
+ * Convert a Boost.Units duration to a std::chrono::duration
+ * \tparam U unit type
+ * \tparam T value type
+ * \param duration Boost.Units based duration
+ * \return std::chrono::duration representative of duration
+ */
+template<typename U, typename T>
+Clock::duration clock_cast(const boost::units::quantity<U, T>& duration)
+{
+    using chrono_type = std::chrono::duration<Duration::value_type>;
+    const Duration quantity { duration };
+    const chrono_type chrono_duration { quantity / si::seconds };
+    return std::chrono::duration_cast<Clock::duration>(chrono_duration);
+}
+
+/**
+ * Convert std::chrono::duration to a Boost.Units quantity
+ * \tparam T type representing ticks
+ * \tparam P tick period type
+ * \param duration std::chrono::duration based duration
+ * \return Boost.Units quantity representing duration
+ */
+template<class T, class P>
+Duration clock_cast(const std::chrono::duration<T, P>& duration)
+{
+    using chrono_type = std::chrono::duration<Duration::value_type>;
+    const auto value = std::chrono::duration_cast<chrono_type>(duration);
+    return value.count() * si::seconds;
+}
+
+} // namespace units
+} // namespace vanetza
+
+#endif /* TIME_HPP_GFC1AX6E */
+