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/recipes/generate-asn1.md

@@ -0,0 +1,47 @@
+Title: Generate code from ASN.1
+
+# How to generate code from ASN.1 definitions
+
+Generating new ASN.1 structs based on ASN.1 definitions can be a daunting task requiring the appropriate ASN.1 compiler.
+In particular, we use the **asn1c** fork maintained by [mouse07410](https://github.com/mouse07410/asn1c).
+
+Though the Vanetza repository comes with various pre-generated ASN.1 messages, you may want to add further or revised versions of these messages.
+In this case, the following steps guide you how to generate code from ASN.1 definitions on your own.
+
+## Prerequisites
+
+We have streamlined the process of installing and using **asn1c** by relying on a Docker container.
+
+1. Install Docker on your system
+2. Build the Docker container from the `vanetza/asn1/Dockerfile` as `vanetza-asn1c:latest`
+
+```bash
+docker build --tag vanetza-asn1c:latest vanetza/asn1
+```
+
+## Generate ASN.1 code
+
+Our CMake build environment comes with a `generate_asn1c` code generation target.
+This target is available if the the `VANETZA_ASN1_WITH_ASN1C` CMake option is enabled.
+When you invoke this target, CMake will run the Docker container for you with suitable arguments.
+You can specify a custom image by modifying the `VANETZA_ASN1C_CONTAINER` CMake cache variable.
+By default, CMake will use the `vanetza-asn1c:latest` image.
+
+1. Update your copy of Vanetza with the new or updated ASN.1 definitions
+2. Update the **CMakeLists.txt** file at **vanetza/asn1** accordingly
+3. Create a fresh build directory and configure CMake with enabled ASN.1 targets
+4. Invoke **asn1c** and compile Vanetza with newly generated code
+
+```bash
+# Create a new build directory at the project's root
+mkdir build.asn1 && cd build.asn1
+
+# Configure the build with enabled ASN.1 targets and retrieval of ISO ASN.1 files
+cmake -DVANETZA_ASN1_WITH_ASN1C=ON -DVANETZA_ASN1_WITH_ISO=ON ..
+
+# Invoke the target calling asn1c
+make generate_asn1c
+
+# Compile Vanetza with newly generated ASN.1 code
+cmake --build .
+```