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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,38 @@
+#!/usr/bin/env python3
+"""Print micrOBU LOG frames without participating in the station-link session."""
+from __future__ import annotations
+
+import argparse
+import time
+
+import serial
+
+from microbu_link.serial_transport import FrameDecoder, FrameType
+
+
+def main():
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument('--port', required=True)
+    parser.add_argument('--duration', type=float, default=15.0)
+    parser.add_argument('--reset', action='store_true', help='pulse EN through USB Serial/JTAG before monitoring')
+    args = parser.parse_args()
+    port = serial.Serial(args.port, 115200, timeout=0.05, write_timeout=1)
+    port.dtr = False
+    port.rts = False
+    if args.reset:
+        port.rts = True
+        time.sleep(0.1)
+        port.rts = False
+    decoder = FrameDecoder()
+    deadline = time.monotonic() + args.duration
+    try:
+        while time.monotonic() < deadline:
+            for frame_type, payload in decoder.feed(port.read(4096)):
+                if frame_type == FrameType.LOG:
+                    print(payload.decode('utf-8', 'replace'), flush=True)
+    finally:
+        port.close()
+
+
+if __name__ == '__main__':
+    main()