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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,33 @@
+#include "transmission.hpp"
+#include <chrono>
+
+namespace vanetza
+{
+namespace dcc
+{
+
+Clock::duration Transmission::channel_occupancy() const
+{
+    using namespace std::chrono;
+
+    // assume 6 Mbps as default data rate
+    const access::DataRateG5* rate = data_rate() ? data_rate() : &access::G5_6Mbps;
+
+    // PHY
+    static const auto phy_preamble = microseconds(32);
+    static const auto phy_signal = microseconds(8);
+
+    // MAC
+    static const std::size_t bytes_epd = 2; // EtherType Protocol Discrimination (no LLC!)
+    static const std::size_t bytes_mac = 34; // 802.11 MAC header
+
+    const std::size_t bytes = rate->data_length(body_length() + bytes_epd + bytes_mac);
+    const double seconds_per_byte = 1.0 / (rate->bytes_per_second());
+    const duration<double> data_duration { bytes * seconds_per_byte };
+
+    return phy_preamble + phy_signal + duration_cast<microseconds>(data_duration);
+}
+
+
+} // namespace dcc
+} // namespace vanetza