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/tools/pki/hexstring.cpp

@@ -0,0 +1,44 @@
+#include "hexstring.hpp"
+#include <boost/algorithm/hex.hpp>
+#include <algorithm>
+#include <cctype>
+#include <iterator>
+
+namespace vanetza
+{
+namespace pki
+{
+
+std::string hexstring(const std::uint8_t* buf, std::size_t len)
+{
+    std::string out;
+    boost::algorithm::hex(buf, buf + len, std::back_inserter(out));
+    return out;
+}
+
+std::string hexstring(const std::string& input)
+{
+    static_assert(sizeof(std::string::value_type) == sizeof(std::uint8_t));
+    return hexstring(reinterpret_cast<const std::uint8_t*>(input.data()), input.size());
+}
+
+std::string hexstring(const ByteBuffer& buffer)
+{
+    return hexstring(buffer.data(), buffer.size());
+}
+
+bool is_valid_hexstring(const std::string& input)
+{
+    return !input.empty() && input.size() % 2 == 0 &&
+        std::all_of(input.begin(), input.end(), [](unsigned char c) { return std::isxdigit(c) != 0; });
+}
+
+std::string parse_hexstring(const std::string& input)
+{
+    std::string out;
+    boost::algorithm::unhex(input, std::back_inserter(out));
+    return out;
+}
+
+} // namespace pki
+} // namespace vanetza