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/security/v2/basic_elements.cpp

@@ -0,0 +1,29 @@
+#include <vanetza/security/v2/basic_elements.hpp>
+#include <algorithm>
+#include <cassert>
+#include <chrono>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v2
+{
+
+Time32 convert_time32(const Clock::time_point& tp)
+{
+    using std::chrono::duration_cast;
+    using seconds = std::chrono::duration<Time32>;
+    return duration_cast<seconds>(tp.time_since_epoch()).count();
+}
+
+Time64 convert_time64(const Clock::time_point& tp)
+{
+    using std::chrono::duration_cast;
+    using microseconds = std::chrono::duration<Time64, std::micro>;
+    return duration_cast<microseconds>(tp.time_since_epoch()).count();
+}
+
+} // namespace v2
+} // namespace security
+} // namespace vanetza