Keep vanetza-idf in obu-firmware, so a plain clone builds the firmware

obu-firmware builds against the vanetza-idf C-ITS library, which until now
came from the colleague's microbu-esp32c5 tree beside the repository and was
not tracked here, so a clone of this repository could not build the firmware
it ships. The library alone is now part of obu-firmware, as
obu-firmware/external/vanetza-idf: their external/vanetza-idf at commit
cf4b99f, unchanged (9775 files; see its PROVENANCE.md). CMake takes it from
there by default; -DVANETZA_IDF_DIR still points the build elsewhere.

The rest of the colleague's tree (their own VAM firmware, PKI tooling,
station-link Python tools, the V2X2MAP bridge) stays out of this repository
and gitignored; nothing is pushed to their repository. NOTES.md, docs/06,
TODO.md and the pcap verifier's usage line point at the new location.

obu-firmware/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 */