Keep vanetza-idf in obu-firmware, so a plain clone builds the firmware

obu-firmware builds against the vanetza-idf C-ITS library, which until now
came from the colleague's microbu-esp32c5 tree beside the repository and was
not tracked here, so a clone of this repository could not build the firmware
it ships. The library alone is now part of obu-firmware, as
obu-firmware/external/vanetza-idf: their external/vanetza-idf at commit
cf4b99f, unchanged (9775 files; see its PROVENANCE.md). CMake takes it from
there by default; -DVANETZA_IDF_DIR still points the build elsewhere.

The rest of the colleague's tree (their own VAM firmware, PKI tooling,
station-link Python tools, the V2X2MAP bridge) stays out of this repository
and gitignored; nothing is pushed to their repository. NOTES.md, docs/06,
TODO.md and the pcap verifier's usage line point at the new location.

obu-firmware/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_
+