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FLASHING.md says how to take a newer version of the colleague's tree (copy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/btp/header.cpp

@@ -0,0 +1,109 @@
+#include "header.hpp"
+#include <vanetza/common/serialization.hpp>
+#include <vanetza/common/serialization_buffer.hpp>
+
+namespace vanetza
+{
+namespace btp
+{
+
+using vanetza::serialize;
+using vanetza::deserialize;
+
+constexpr std::size_t HeaderA::length_bytes;
+constexpr std::size_t HeaderB::length_bytes;
+
+void serialize(OutputArchive& ar, const HeaderA& hdr)
+{
+    serialize(ar, hdr.destination_port);
+    serialize(ar, hdr.source_port);
+}
+
+void deserialize(InputArchive& ar, HeaderA& hdr)
+{
+    deserialize(ar, hdr.destination_port);
+    deserialize(ar, hdr.source_port);
+}
+
+void serialize(OutputArchive& ar, const HeaderB& hdr)
+{
+    serialize(ar, hdr.destination_port);
+    serialize(ar, hdr.destination_port_info);
+}
+
+void deserialize(InputArchive& ar, HeaderB& hdr)
+{
+    deserialize(ar, hdr.destination_port);
+    deserialize(ar, hdr.destination_port_info);
+}
+
+HeaderA parse_btp_a(CohesivePacket& packet)
+{
+    HeaderA hdr;
+    deserialize_from_range(hdr, packet[OsiLayer::Transport]);
+    packet.set_boundary(OsiLayer::Transport, btp::HeaderA::length_bytes);
+    return hdr;
+}
+
+HeaderA parse_btp_a(ChunkPacket& packet)
+{
+    HeaderA hdr;
+    ByteBuffer tmp;
+    packet[OsiLayer::Transport].convert(tmp);
+    deserialize_from_buffer(hdr, tmp);
+    return hdr;
+}
+
+HeaderA parse_btp_a(PacketVariant& packet)
+{
+    struct parse_btp_visitor : public boost::static_visitor<HeaderA>
+    {
+        HeaderA operator()(CohesivePacket& packet) {
+            return parse_btp_a(packet);
+        }
+
+        HeaderA operator()(ChunkPacket& packet) {
+            return parse_btp_a(packet);
+        }
+    };
+
+    parse_btp_visitor visitor;
+    return boost::apply_visitor(visitor, packet);
+}
+
+HeaderB parse_btp_b(CohesivePacket& packet)
+{
+    HeaderB hdr;
+    deserialize_from_range(hdr, packet[OsiLayer::Transport]);
+    packet.set_boundary(OsiLayer::Transport, btp::HeaderB::length_bytes);
+    return hdr;
+}
+
+HeaderB parse_btp_b(ChunkPacket& packet)
+{
+    HeaderB hdr;
+    ByteBuffer tmp;
+    packet[OsiLayer::Transport].convert(tmp);
+    deserialize_from_buffer(hdr, tmp);
+    return hdr;
+}
+
+HeaderB parse_btp_b(PacketVariant& packet)
+{
+    struct parse_btp_visitor : public boost::static_visitor<HeaderB>
+    {
+        HeaderB operator()(CohesivePacket& packet) {
+            return parse_btp_b(packet);
+        }
+
+        HeaderB operator()(ChunkPacket& packet) {
+            return parse_btp_b(packet);
+        }
+    };
+
+    parse_btp_visitor visitor;
+    return boost::apply_visitor(visitor, packet);
+}
+
+} // namespace btp
+} // namespace vanetza