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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,35 @@
+#include "sequence_number.hpp"
+
+namespace vanetza
+{
+namespace geonet
+{
+
+bool SequenceNumber::operator<(SequenceNumber other) const
+{
+    return ((other.m_number > m_number && other.m_number - m_number <= SequenceNumber::max/2) ||
+        (m_number > other.m_number && m_number - other.m_number > SequenceNumber::max/2));
+}
+
+SequenceNumber SequenceNumber::operator++(int)
+{
+    SequenceNumber tmp = *this;
+    ++m_number;
+    return tmp;
+}
+
+void serialize(const SequenceNumber& sn, OutputArchive& ar)
+{
+    serialize(host_cast(static_cast<SequenceNumber::value_type>(sn)), ar);
+}
+
+void deserialize(SequenceNumber& sn, InputArchive& ar)
+{
+    SequenceNumber::value_type tmp = 0;
+    deserialize(tmp, ar);
+    sn = SequenceNumber(tmp);
+}
+
+} // namespace geonet
+} // namespace vanetza
+