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.cpp

@@ -0,0 +1,51 @@
+#include <vanetza/security/v2/int_x.hpp>
+#include <vanetza/security/v2/length_coding.hpp>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v2
+{
+
+void IntX::set(integer_type x)
+{
+    m_value = x;
+}
+
+ByteBuffer IntX::encode() const
+{
+    return encode_length(m_value);
+}
+
+boost::optional<IntX> IntX::decode(const ByteBuffer& buffer)
+{
+    std::tuple<ByteBuffer::const_iterator, std::uintmax_t> decoded = decode_length(buffer);
+    if (std::get<0>(decoded) != buffer.begin()) {
+        IntX result;
+        result.set(std::get<1>(decoded));
+        return result;
+    }
+    return boost::none;
+}
+
+size_t get_size(IntX intx)
+{
+    return length_coding_size(intx.get());
+}
+
+void serialize(OutputArchive& ar, const IntX& intx)
+{
+    serialize_length(ar, intx.get());
+}
+
+size_t deserialize(InputArchive& ar, IntX& intx)
+{
+    const auto size = deserialize_length(ar);
+    intx.set(size);
+    return get_size(intx);
+}
+
+} // namespace v2
+} // namespace security
+} // namespace vanetza