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/common/byte_buffer_sink.hpp

@@ -0,0 +1,29 @@
+#ifndef BYTE_BUFFER_SINK_HPP_XQSBHPDQ
+#define BYTE_BUFFER_SINK_HPP_XQSBHPDQ
+
+#include <vanetza/common/byte_buffer.hpp>
+#include <boost/iostreams/categories.hpp>
+#include <ios>
+
+namespace vanetza
+{
+
+class byte_buffer_sink
+{
+public:
+    typedef char char_type;
+    typedef boost::iostreams::sink_tag category;
+    static_assert(sizeof(char_type) == sizeof(ByteBuffer::value_type),
+            "size mismatch of char_type and ByteBuffer::value_type");
+
+    byte_buffer_sink(ByteBuffer& buffer);
+    std::streamsize write(const char_type* s, std::streamsize n);
+
+private:
+    ByteBuffer& m_buffer;
+};
+
+} // namespace vanetza
+
+#endif /* BYTE_BUFFER_SINK_HPP_XQSBHPDQ */
+