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/serialization.cpp

@@ -0,0 +1,65 @@
+#include <vanetza/security/v2/length_coding.hpp>
+#include <vanetza/security/v2/serialization.hpp>
+#include <cassert>
+#include <limits>
+#include <stdexcept>
+#include <type_traits>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v2
+{
+
+template<typename T>
+std::size_t trim_size_impl(T in, typename std::enable_if<std::is_same<T, std::size_t>::value>::type* = nullptr)
+{
+    return in;
+}
+
+template<typename T>
+std::size_t trim_size_impl(T in, typename std::enable_if<!std::is_same<T, std::size_t>::value>::type* = nullptr)
+{
+    if (in > std::numeric_limits<std::size_t>::max()) {
+        throw std::overflow_error("given size exceeds limits of std::size_t");
+    }
+    return static_cast<std::size_t>(in);
+}
+
+std::size_t trim_size(std::uintmax_t in)
+{
+    return trim_size_impl(in);
+}
+
+
+void serialize_length(OutputArchive& ar, std::uintmax_t length)
+{
+    ByteBuffer buf;
+    buf = encode_length(length);
+    for (auto it = buf.begin(); it != buf.end(); it++) {
+        ar << *it;
+    }
+}
+
+std::uintmax_t deserialize_length(InputArchive& ar)
+{
+    ByteBuffer buf(1);
+    ar >> buf[0];
+    const size_t leading = count_leading_ones(buf[0]);
+    buf.resize(leading + 1);
+    for (size_t c = 1; c <= leading; ++c) {
+        ar >> buf[c];
+    }
+    auto tup = decode_length(buf);
+    if (std::get<0>(tup) != buf.begin()) {
+        return std::get<1>(tup);
+    } else {
+        ar.fail(InputArchive::ErrorCode::ConstraintViolation);
+        return 0;
+    }
+}
+
+} // namespace v2
+} // namespace security
+} // namespace vanetza