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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,82 @@
+#ifndef PACKET_VARIANT_HPP_LILZ0UWN
+#define PACKET_VARIANT_HPP_LILZ0UWN
+
+#include <vanetza/common/byte_view.hpp>
+#include <vanetza/common/serialization.hpp>
+#include <vanetza/net/chunk_packet.hpp>
+#include <vanetza/net/cohesive_packet.hpp>
+#include <boost/variant.hpp>
+
+namespace vanetza
+{
+
+using PacketVariant = boost::variant<ChunkPacket, CohesivePacket>;
+
+inline std::size_t
+size(const CohesivePacket& packet, OsiLayer from, OsiLayer to)
+{
+    return packet.size(from, to);
+}
+
+inline std::size_t
+size(const CohesivePacket& packet, OsiLayer layer)
+{
+    return packet.size(layer);
+}
+
+inline std::size_t
+size(const ChunkPacket& packet, OsiLayer from, OsiLayer to)
+{
+    return packet.size(from, to);
+}
+
+inline std::size_t
+size(const ChunkPacket& packet, OsiLayer layer)
+{
+    return packet[layer].size();
+}
+
+/**
+ * Create a view of a packet's bytes assigned to a certain layer
+ * \param packet
+ * \param layer
+ * \return a byte view, possibly empty
+ */
+byte_view_range create_byte_view(const ChunkPacket&, OsiLayer);
+byte_view_range create_byte_view(const CohesivePacket&, OsiLayer);
+
+/**
+ * Create a byte view for a sub-range of layers
+ * \param packet
+ * \param from first byte will be from this layer
+ * \param to last byte will be from this layer
+ * \return a byte view
+ */
+byte_view_range create_byte_view(const ChunkPacket& packet, OsiLayer from, OsiLayer to);
+byte_view_range create_byte_view(const CohesivePacket& packet, OsiLayer from, OsiLayer to);
+
+/**
+ * Serialize a whole packet from physical to application layer
+ * \param archive destination archive
+ * \param packet source packet
+ */
+void serialize(OutputArchive&, const ChunkPacket&);
+void serialize(OutputArchive&, const CohesivePacket&);
+
+} // namespace vanetza
+
+namespace boost
+{
+
+// PacketVariant is only a type alias, so we need this quirk for proper name lookup
+std::size_t size(const vanetza::PacketVariant&, vanetza::OsiLayer from, vanetza::OsiLayer to);
+std::size_t size(const vanetza::PacketVariant&, vanetza::OsiLayer);
+std::size_t size(const vanetza::PacketVariant&);
+vanetza::byte_view_range create_byte_view(const vanetza::PacketVariant&, vanetza::OsiLayer);
+vanetza::byte_view_range create_byte_view(const vanetza::PacketVariant&, vanetza::OsiLayer from, vanetza::OsiLayer to);
+void serialize(vanetza::OutputArchive&, const vanetza::PacketVariant&);
+
+} // namespace boost
+
+#endif /* PACKET_VARIANT_HPP_LILZ0UWN */
+