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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,73 @@
+#ifndef INT_X_HPP_RW3TJBBI
+#define INT_X_HPP_RW3TJBBI
+
+#include <vanetza/common/byte_buffer.hpp>
+#include <vanetza/security/v2/serialization.hpp>
+#include <boost/operators.hpp>
+#include <boost/optional.hpp>
+#include <cstdint>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v2
+{
+
+/// IntX specified in TS 103 097 v1.2.1, section 4.2.1
+class IntX :
+    public boost::equality_comparable<IntX>,
+    public boost::equality_comparable<IntX, std::uintmax_t>
+{
+public:
+    using integer_type = std::uintmax_t;
+    constexpr IntX() : m_value(0) {}
+    constexpr explicit IntX(integer_type x) : m_value(x) {}
+
+    void set(integer_type x);
+    constexpr integer_type get() const { return m_value; }
+
+    constexpr bool operator==(const IntX& other) const
+    {
+        return m_value == other.m_value;
+    }
+
+    constexpr bool operator==(integer_type other) const
+    {
+        return m_value == other;
+    }
+
+    ByteBuffer encode() const;
+    static boost::optional<IntX> decode(const ByteBuffer&);
+
+private:
+    integer_type m_value;
+};
+
+/**
+ * \brief Serializes an IntX into a binary archive
+ * \param ar to serialize in
+ * \param intx to serialize
+ */
+void serialize(OutputArchive&, const IntX&);
+
+/**
+ * \brief Deserializes an IntX from a binary archive
+ * \param ar with a serialized IntX at the beginning
+ * \param intx to deserialize
+ * \return size of the deserialized IntX
+ */
+size_t deserialize(InputArchive&, IntX&);
+
+/**
+ * \brief Calculates size of an IntX
+ * \param intx
+ * \return number of octets needed to serialize the IntX
+ */
+size_t get_size(IntX);
+
+} // namespace v2
+} // namespace security
+} // namespace vanetza
+
+#endif /* INT_X_HPP_RW3TJBBI */