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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,55 @@
+#ifndef BYTE_BUFFER_HPP_7NOEQO4F
+#define BYTE_BUFFER_HPP_7NOEQO4F
+
+#include <cstdint>
+#include <type_traits>
+#include <vector>
+
+namespace vanetza
+{
+
+typedef std::vector<uint8_t> ByteBuffer;
+
+/**
+ * Cast byte buffer to a POD object
+ * \param buffer byte buffer containing requested object
+ * \return pointer to object or nullptr if cast is impossible
+ */
+template<typename MASK>
+MASK* buffer_cast(ByteBuffer& buffer)
+{
+    static_assert(std::is_standard_layout<MASK>::value, "MASK has to be standard layout type");
+    static_assert(std::is_trivially_copyable<MASK>::value, "MASK has to be trivially copyable");
+    static_assert(std::is_object<MASK>::value, "MASK has to be an object");
+
+    MASK* mask = nullptr;
+    if (sizeof(MASK) <= buffer.size()) {
+        mask = reinterpret_cast<MASK*>(&buffer[0]);
+    }
+    return mask;
+}
+
+template<typename MASK>
+const MASK* buffer_cast(const ByteBuffer& buffer)
+{
+    // const_cast is safe, const qualifier is added to return type
+    return buffer_cast<MASK>(const_cast<ByteBuffer&>(buffer));
+}
+
+/**
+ * Create byte buffer with copy of POD object
+ * \param obj POD object
+ * \return ByteBuffer object with copy
+ */
+template<class T>
+ByteBuffer buffer_copy(const T& object)
+{
+    static_assert(std::is_standard_layout<T>::value, "T has to be standard layout type");
+    auto ptr = reinterpret_cast<const uint8_t*>(&object);
+    return ByteBuffer(ptr, ptr + sizeof(T));
+}
+
+} // namespace vanetza
+
+#endif /* BYTE_BUFFER_HPP_7NOEQO4F */
+