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/length_coding.hpp

@@ -0,0 +1,49 @@
+#ifndef LENGTH_CODING_HPP_UQ1OIDUN
+#define LENGTH_CODING_HPP_UQ1OIDUN
+
+#include <vanetza/common/byte_buffer.hpp>
+#include <boost/range/iterator_range.hpp>
+#include <cstdint>
+#include <tuple>
+
+namespace vanetza
+{
+namespace security
+{
+namespace v2
+{
+
+/**
+ * Calculate length coding for variable length fields
+ * \param length Data field length in bytes, e.g. size of a buffer
+ * \return byte buffer containing encoded length, prepend to data
+ */
+ByteBuffer encode_length(std::uintmax_t length);
+
+/**
+ * Extract length information
+ * \param buffer Buffer with input data, shall start with first byte of encoded length
+ * \return iterator pointing at start of payload buffer (begin indicates error) and its length
+ */
+std::tuple<ByteBuffer::const_iterator, std::uintmax_t> decode_length(const ByteBuffer& buffer);
+
+/**
+ * Count number of leading one bits
+ * \param a byte
+ * \return number of leadings ones in given byte
+ */
+std::size_t count_leading_ones(std::uint8_t);
+
+/**
+ * Determines the number of bytes, needed to store a given size
+ * \param size
+ * \return number of bytes needed to store length
+ */
+std::size_t length_coding_size(std::uintmax_t);
+
+} // namespace v2
+} // namespace security
+} // namespace vanetza
+
+#endif /* LENGTH_CODING_HPP_UQ1OIDUN */
+