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/sockaddr.cpp

@@ -0,0 +1,17 @@
+#include "sockaddr.hpp"
+#include "mac_address.hpp"
+#include <algorithm>
+#include <cassert>
+#include <net/ethernet.h>
+#include <netpacket/packet.h>
+
+namespace vanetza
+{
+
+void assign(sockaddr_ll& sockaddr, const MacAddress& mac)
+{
+    assert(ETHER_ADDR_LEN == mac.octets.size());
+    std::copy_n(mac.octets.begin(), mac.octets.size(), sockaddr.sll_addr);
+}
+
+} // namespace vanetza