Keep vanetza-idf in obu-firmware, so a plain clone builds the firmware

obu-firmware builds against the vanetza-idf C-ITS library, which until now
came from the colleague's microbu-esp32c5 tree beside the repository and was
not tracked here, so a clone of this repository could not build the firmware
it ships. The library alone is now part of obu-firmware, as
obu-firmware/external/vanetza-idf: their external/vanetza-idf at commit
cf4b99f, unchanged (9775 files; see its PROVENANCE.md). CMake takes it from
there by default; -DVANETZA_IDF_DIR still points the build elsewhere.

The rest of the colleague's tree (their own VAM firmware, PKI tooling,
station-link Python tools, the V2X2MAP bridge) stays out of this repository
and gitignored; nothing is pushed to their repository. NOTES.md, docs/06,
TODO.md and the pcap verifier's usage line point at the new location.

obu-firmware/external/vanetza-idf/docs/how-to-build.md

@@ -0,0 +1,47 @@
+# How to build
+
+Building Vanetza is accomplished by the [CMake](https://cmake.org) build system. Hence, CMake needs to be available on the build host.
+
+## Prerequisites
+
+You need following tools and libraries on your system for compiling Vanetza:
+
+* C++11 compatible compiler, e.g. [GNU GCC](https://gcc.gnu.org) or [Clang](http://clang.llvm.org)
+* [CMake](https://cmake.org) 3.12 or higher
+* [Boost](https://www.boost.org) 1.58 or higher
+* [GeographicLib](http://geographiclib.sourceforge.net) 1.37 or higher
+* [Crypto++](https://www.cryptopp.com) 5.6.1 or higher
+
+If OpenSSL (1.0 or 1.1) or LibreSSL is available on your system, an alternative security backend implementation is compiled along with the Crypto++ based backend.
+See `security::Backend` and `security::create_backend` for more details.
+
+!!! hint
+    If you are building on Ubuntu 16.04 or 18.04, please note that these Ubuntu releases ship outdated CMake versions unsuitable for Vanetza.
+    Have a look at the [Kitware APT Repository](https://apt.kitware.com), which lists some of your options for installing an up-to-date CMake version.
+
+
+## Compilation
+
+Following command line snippet demonstrates the build process using a generated Makefile.
+Other CMake generators and build directory setups can be used as well.
+
+    :::shell
+    cd vanetza
+    mkdir build && cd build
+    cmake ..
+    make
+
+
+## Unit tests
+
+Vanetza comes with many unit tests covering the most critical parts.
+When the CMake option `BUILD_TESTS` is enabled, the [Google Test](https://github.com/google/googletest/) sources are downloaded by the build system automatically.
+The built test cases are standalone executables located in the *tests* subdirectory of your build directory.
+For running all test cases, I recommend to invoke `ctest` in your build directory.
+
+
+## Integrating Vanetza
+
+Vanetza is primarily a library project intended for integration by other projects, e.g. V2X simulation tools such as [Artery](https://github.com/riebl/artery).
+Projects using CMake can integrate Vanetza most easily by calling `find_package(Vanetza)` and then refer to the imported Vanetza targets, such as `Vanetza::vanetza`.
+