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FLASHING.md says how to take a newer version of the colleague's tree (copy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