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FLASHING.md says how to take a newer version of the colleague's tree (copy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/tools/fuzz-harness/docker.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+set -eu
+HARNESS_DIR=$(realpath $(dirname $0))
+SOURCE_DIR=$HARNESS_DIR/../..
+
+docker build $HARNESS_DIR
+IMAGE=$(docker build -q $HARNESS_DIR)
+
+mkdir -p $HARNESS_DIR/output
+docker run --rm -it \
+    --security-opt seccomp=unconfined \
+    -v$SOURCE_DIR:/source:ro \
+    -v$HARNESS_DIR/input:/input:ro \
+    -v$HARNESS_DIR/output:/output \
+    -e HOST_USER_ID=$(id -u) -e HOST_GROUP_ID=$(id -g) \
+    $IMAGE