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/filesystem.cpp

@@ -0,0 +1,62 @@
+#include "filesystem.hpp"
+#include <vanetza/common/byte_buffer.hpp>
+#include <fstream>
+#include <stdexcept>
+#include <system_error>
+
+namespace vanetza
+{
+namespace pki
+{
+
+namespace
+{
+
+void write_file(const std::filesystem::path& path, const ByteBuffer& buffer)
+{
+    std::ofstream ofs(path, std::ios::binary | std::ios::trunc);
+    if (!ofs) {
+        throw std::runtime_error("could not open " + path.string() + " for writing");
+    }
+    ofs.write(reinterpret_cast<const char*>(buffer.data()), buffer.size());
+    ofs.flush();
+    if (!ofs) {
+        throw std::runtime_error("could not write " + path.string());
+    }
+}
+
+} // namespace
+
+void write(const std::filesystem::path& path, const ByteBuffer& buffer)
+{
+    // Write to temporary file first and atomically rename it later
+    std::filesystem::path tmp = path;
+    tmp += ".tmp";
+
+    try {
+        write_file(tmp, buffer);
+        std::filesystem::rename(tmp, path);
+    } catch (...) {
+        // clean up temporary file
+        std::error_code ec;
+        std::filesystem::remove(tmp, ec);
+        throw;
+    }
+}
+
+ByteBuffer read(const std::filesystem::path& path)
+{
+    std::ifstream ifs(path, std::ios::binary);
+    if (!ifs) {
+        throw std::runtime_error("could not open " + path.string() + " for reading");
+    }
+
+    ByteBuffer buffer { std::istreambuf_iterator<char>(ifs), std::istreambuf_iterator<char>() };
+    if (ifs.bad()) {
+        throw std::runtime_error("I/O error while reading " + path.string());
+    }
+    return buffer;
+}
+
+} // namespace pki
+} // namespace vanetza