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FLASHING.md says how to take a newer version of the colleague's tree (copy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/station-link/python/sut_tcp_relay.py

@@ -0,0 +1,32 @@
+#!/usr/bin/env python3
+"""stdin/stdout <-> TCP relay: the SUT executable an ETSI adapter forks inside WSL, forwarding
+the hex-line protocol to sut_bridge.py --listen on the Windows side (which owns the COM port).
+
+  run_etsi.py ... --sut /usr/bin/python3 --sut-args "/root/sut_tcp_relay.py --connect 172.x.x.x:7788"
+"""
+import argparse
+import socket
+import sys
+
+
+def main():
+    p = argparse.ArgumentParser(description=__doc__)
+    p.add_argument('--connect', required=True, help='host:port of sut_bridge.py --listen')
+    args = p.parse_args()
+    host, port = args.connect.rsplit(':', 1)
+    with socket.create_connection((host, int(port))) as connection:
+        reader = connection.makefile('rb')
+        for line in sys.stdin:
+            line = line.strip()
+            if not line:
+                continue
+            connection.sendall(line.encode() + b'\n')
+            reply = reader.readline()
+            if not reply:
+                sys.exit(2)
+            sys.stdout.write(reply.decode().strip() + '\n')
+            sys.stdout.flush()
+
+
+if __name__ == '__main__':
+    main()