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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,26 @@
+set(CXX_SOURCES
+    buffer_packet.cpp
+    chunk_packet.cpp
+    cohesive_packet.cpp
+    ethernet_header.cpp
+    mac_address.cpp
+    packet_variant.cpp
+    packet.cpp
+    proxy_header.cpp
+)
+
+set(CXX_SOURCES_POSIX
+    io_vector.cpp
+    sockaddr.cpp
+)
+
+option(VANETZA_NET_WITH_POSIX "Build network module with POSIX extension" ${UNIX})
+if(VANETZA_NET_WITH_POSIX)
+    list(APPEND CXX_SOURCES ${CXX_SOURCES_POSIX})
+endif()
+
+add_vanetza_component(net ${CXX_SOURCES})
+target_link_libraries(net PUBLIC common)
+
+add_test_subdirectory(tests)
+