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FLASHING.md says how to take a newer version of the colleague's tree (copy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