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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,51 @@
+#include "ethernet_header.hpp"
+#include <algorithm>
+#include <cassert>
+#include <stdexcept>
+
+namespace vanetza
+{
+
+ByteBuffer create_ethernet_header(const MacAddress& dest, const MacAddress& src, uint16be_t proto)
+{
+    ByteBuffer buffer;
+    buffer.reserve(EthernetHeader::length_bytes);
+    auto inserter = std::back_inserter(buffer);
+    std::copy(dest.octets.begin(), dest.octets.end(), inserter);
+    std::copy(src.octets.begin(), src.octets.end(), inserter);
+    uint16_t host_proto = proto.host();
+    inserter = (host_proto >> 8) & 0xff;
+    inserter = host_proto & 0xff;
+    assert(buffer.size() == EthernetHeader::length_bytes);
+    return buffer;
+}
+
+ByteBuffer create_ethernet_header(const EthernetHeader& hdr)
+{
+    return create_ethernet_header(hdr.destination, hdr.source, hdr.type);
+}
+
+EthernetHeader decode_ethernet_header(ByteBuffer::const_iterator begin, ByteBuffer::const_iterator end)
+{
+    EthernetHeader hdr;
+    const std::size_t buflen = std::distance(begin, end);
+    if (buflen < EthernetHeader::length_bytes) {
+        throw std::runtime_error("buffer is too short for EthernetHeader decoding");
+    } else {
+        std::copy_n(begin, MacAddress::length_bytes, hdr.destination.octets.begin());
+        begin += MacAddress::length_bytes;
+        std::copy_n(begin, MacAddress::length_bytes, hdr.source.octets.begin());
+        begin += MacAddress::length_bytes;
+        uint16_t proto = (begin[0] << 8) | begin[1];
+        hdr.type = host_cast(proto);
+    }
+    return hdr;
+}
+
+EthernetHeader decode_ethernet_header(const ByteBuffer& buf)
+{
+    return decode_ethernet_header(buf.begin(), buf.end());
+}
+
+} // namespace vanetza
+