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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,57 @@
+#ifndef ETHERNET_HEADER_HPP_
+#define ETHERNET_HEADER_HPP_
+
+#include <vanetza/common/byte_buffer.hpp>
+#include <vanetza/common/byte_order.hpp>
+#include <vanetza/net/mac_address.hpp>
+#include <cstddef>
+
+namespace vanetza
+{
+
+/**
+ * Get length of ethernet header in bytes
+ * \param header length in bytes
+ */
+constexpr std::size_t ethernet_header_length()
+{
+    return 2 * sizeof(MacAddress::octets) + sizeof(uint16_t);
+}
+
+/**
+ * Link-level header of type Ethernet II
+ *
+ * EthernetHeader is suitable for Linux raw packet sockets, see man 7 packet
+ */
+class EthernetHeader
+{
+public:
+    using EtherType = uint16be_t;
+    static constexpr std::size_t length_bytes = ethernet_header_length();
+
+    MacAddress destination;
+    MacAddress source;
+    EtherType type;
+};
+
+/**
+ * Create a byte buffer containing an ethernet header
+ * \param dest Destination MAC address
+ * \param src Source MAC address
+ * \param proto EtherType number
+ */
+ByteBuffer create_ethernet_header(const MacAddress& dest, const MacAddress& src, uint16be_t proto);
+ByteBuffer create_ethernet_header(const EthernetHeader&);
+
+/**
+ * Parse ethernet header from byte buffer
+ * \param buffer byte buffer (throws exception if too short)
+ * \return ethernet header
+ */
+EthernetHeader decode_ethernet_header(const ByteBuffer&);
+EthernetHeader decode_ethernet_header(ByteBuffer::const_iterator, ByteBuffer::const_iterator);
+
+} // namespace vanetza
+
+#endif // ETHERNET_HEADER_HPP_
+