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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,39 @@
+#ifndef BYTE_BUFFER_SOURCE_HPP_6IBYOI0T
+#define BYTE_BUFFER_SOURCE_HPP_6IBYOI0T
+
+#include <vanetza/common/byte_buffer.hpp>
+#include <boost/iostreams/categories.hpp>
+#include <boost/range/iterator_range.hpp>
+
+namespace vanetza
+{
+
+/**
+ * Implementation of boost::iostreams' source device concept.
+ * Allows to use a byte buffer as input stream.
+ */
+class byte_buffer_source
+{
+public:
+    typedef char char_type;
+    typedef boost::iostreams::source_tag category;
+    typedef ByteBuffer::const_iterator iterator;
+    typedef boost::iterator_range<iterator> range;
+
+    static_assert(sizeof(char_type) == sizeof(ByteBuffer::value_type),
+            "size mismatch of char_type and ByteBuffer::value_type");
+
+    byte_buffer_source(const ByteBuffer& buffer);
+    byte_buffer_source(range r);
+    byte_buffer_source(iterator begin, iterator end);
+    std::streamsize read(char_type* buf, std::streamsize n);
+
+private:
+    iterator m_begin;
+    iterator m_end;
+};
+
+} // namespace vanetza
+
+#endif /* BYTE_BUFFER_SOURCE_HPP_6IBYOI0T */
+