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FLASHING.md says how to take a newer version of the colleague's tree (copy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/CMakeLists.txt

@@ -0,0 +1,48 @@
+set(CXX_SOURCES
+    address.cpp
+    areas.cpp
+    basic_header.cpp
+    beacon_header.cpp
+    cbf_packet_buffer.cpp
+    cbf_counter.cpp
+    cbf_packet_identifier.cpp
+    cbr_aggregator.cpp
+    common_header.cpp
+    data_confirm.cpp
+    data_request.cpp
+    dcc_field.cpp
+    dcc_information_sharing.cpp
+    dcc_mco_field.cpp
+    duplicate_packet_list.cpp
+    gbc_gac_header.cpp
+    gbc_header.cpp
+    gbc_memory.cpp
+    header_type.cpp
+    header_variant.cpp
+    indication_context.cpp
+    mib.cpp
+    sequence_number.cpp
+    lifetime.cpp
+    location_table.cpp
+    next_hop.cpp
+    packet_buffer.cpp
+    parser.cpp
+    pdu.cpp
+    pdu_conversion.cpp
+    position_updater.cpp
+    position_vector.cpp
+    repeater.cpp
+    router.cpp
+    secured_pdu.cpp
+    shb_header.cpp
+    timestamp.cpp
+    traffic_class.cpp
+    tsb_header.cpp
+    variant_pdu.cpp
+)
+
+add_vanetza_component(geonet ${CXX_SOURCES})
+target_link_libraries(geonet PUBLIC Boost::date_time)
+target_link_libraries(geonet PUBLIC dcc geodesy net security)
+
+add_test_subdirectory(tests)