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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,42 @@
+#ifndef SEQUENCE_NUMBER_HPP_HEO4A3XC
+#define SEQUENCE_NUMBER_HPP_HEO4A3XC
+
+#include <vanetza/geonet/serialization.hpp>
+#include <boost/operators.hpp>
+#include <cstdint>
+#include <limits>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+class SequenceNumber :
+    public boost::totally_ordered<SequenceNumber>,
+    public boost::additive<SequenceNumber>
+{
+public:
+    using value_type = uint16_t;
+    static constexpr value_type max = std::numeric_limits<value_type>::max();
+
+    SequenceNumber() : m_number(0) {}
+    explicit SequenceNumber(value_type number) : m_number(number) {}
+    explicit operator value_type() const { return m_number; }
+    bool operator<(SequenceNumber other) const;
+    bool operator==(SequenceNumber other) const { return m_number == other.m_number; }
+    void operator+=(SequenceNumber other) { m_number += other.m_number; }
+    void operator-=(SequenceNumber other) { m_number -= other.m_number; }
+    SequenceNumber operator++(int);
+
+private:
+    value_type m_number;
+};
+
+void serialize(const SequenceNumber&, OutputArchive&);
+void deserialize(SequenceNumber&, InputArchive&);
+
+} // namespace geonet
+} // namespace vanetza
+
+#endif /* SEQUENCE_NUMBER_HPP_HEO4A3XC */
+