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FLASHING.md says how to take a newer version of the colleague's tree (copy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