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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,21 @@
+#!/bin/bash
+source /etc/os-release
+export DEBIAN_FRONTEND=noninteractive
+
+install_kitware_repo() {
+    GPG_KEYFILE=/usr/share/keyrings/kitware-archive-keyring.gpg
+    wget -O - https://apt.kitware.com/keys/kitware-archive-latest.asc 2>/dev/null | gpg --dearmor > $GPG_KEYFILE
+    echo "deb [signed-by=$GPG_KEYFILE] https://apt.kitware.com/ubuntu/ ${UBUNTU_CODENAME} main" > /etc/apt/sources.list.d/kitware.list
+    echo "Added Kitware repository for Ubuntu $UBUNTU_CODENAME"
+}
+
+if [[ "${UBUNTU_CODENAME}" == "xenial" ]]; then
+    apt-get update && apt-get install -y apt-transport-https wget
+    install_kitware_repo
+elif [[ "${UBUNTU_CODENAME}" == "bionic" ]]; then
+    apt-get update && apt-get install -y gpg wget
+    install_kitware_repo
+else
+    # Ubuntu focal and later ship with a sufficiently new CMake version
+    echo "No need to add Kitware repository for Ubuntu $UBUNTU_CODENAME"
+fi