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FLASHING.md says how to take a newer version of the colleague's tree (copy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/geonet/basic_header.cpp

@@ -0,0 +1,71 @@
+#include <vanetza/geonet/basic_header.hpp>
+#include <vanetza/geonet/data_request.hpp>
+#include <vanetza/geonet/serialization.hpp>
+
+namespace vanetza
+{
+namespace geonet
+{
+
+constexpr std::size_t BasicHeader::length_bytes;
+
+BasicHeader::BasicHeader() :
+    version(0),
+    next_header(NextHeaderBasic::Any),
+    reserved(0),
+    hop_limit(0)
+{
+}
+
+BasicHeader::BasicHeader(const MIB& mib) :
+    version(mib.itsGnProtocolVersion),
+    next_header(NextHeaderBasic::Any),
+    reserved(0),
+    lifetime(mib.itsGnDefaultPacketLifetime),
+    hop_limit(mib.itsGnDefaultHopLimit)
+{
+}
+
+BasicHeader::BasicHeader(const DataRequest& request, const MIB& mib) :
+    BasicHeader(mib)
+{
+    if (mib.itsGnSecurity) {
+        next_header = NextHeaderBasic::Secured;
+    } else {
+        next_header = NextHeaderBasic::Common;
+    }
+
+    lifetime = request.maximum_lifetime;
+    hop_limit = request.max_hop_limit;
+}
+
+BasicHeader::BasicHeader(const ShbDataRequest& request, const MIB& mib) :
+    BasicHeader(static_cast<const DataRequest&>(request), mib)
+{
+    hop_limit = 1;
+}
+
+void serialize(const BasicHeader& hdr, OutputArchive& ar)
+{
+    uint8_t versionAndNextHeader = hdr.version.raw();
+    versionAndNextHeader <<= 4;
+    versionAndNextHeader |= static_cast<uint8_t>(hdr.next_header) & 0x0f;
+    serialize(host_cast(versionAndNextHeader), ar);
+    serialize(host_cast(hdr.reserved), ar);
+    serialize(hdr.lifetime, ar);
+    serialize(host_cast(hdr.hop_limit), ar);
+}
+
+void deserialize(BasicHeader& hdr, InputArchive& ar)
+{
+    uint8_t versionAndNextHeader;
+    deserialize(versionAndNextHeader, ar);
+    hdr.version = versionAndNextHeader >> 4;
+    hdr.next_header = static_cast<NextHeaderBasic>(versionAndNextHeader & 0x0f);
+    deserialize(hdr.reserved, ar);
+    deserialize(hdr.lifetime, ar);
+    deserialize(hdr.hop_limit, ar);
+}
+
+} // namespace geonet
+} // namespace vanetza