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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,43 @@
+#ifndef BASIC_ELEMENTS_HPP_RALCTYHI
+#define BASIC_ELEMENTS_HPP_RALCTYHI
+
+#include <vanetza/common/clock.hpp>
+#include <vanetza/security/hashed_id.hpp>
+#include <cstdint>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v2
+{
+
+using Time64 = uint64_t;
+using Time32 = uint32_t;
+
+/// Time64WithStandardDeviation specified in TS 103 097 v1.2.1, section 4.2.16
+struct Time64WithStandardDeviation
+{
+    Time64 time64;
+    uint8_t log_std_dev;
+};
+
+/**
+ * Convert time point to time stamp
+ * \param tp time point
+ * \return time stamp with second accuracy
+ */
+Time32 convert_time32(const Clock::time_point& tp);
+
+/**
+ * Convert time point to time stamp
+ * \param tp time point
+ * \return time stamp with microsecond accuracy
+ */
+Time64 convert_time64(const Clock::time_point& tp);
+
+} // namespace v2
+} // namespace security
+} // namespace vanetza
+
+#endif /* BASIC_ELEMENTS_HPP_RALCTYHI */