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/cmake/FindCohda.cmake

@@ -0,0 +1,20 @@
+find_path(COHDA_ROOT
+    NAMES "ieee1609/app/lib1609/include/lib1609.h" "cohda/kernel/include/linux/cohda/llc/llc-api.h"
+    PATHS "/home/duser"
+    PATH_SUFFIXES mk5 mk2
+    CMAKE_FIND_ROOT_PATH_BOTH
+    DOC "Cohda SDK")
+
+set(COHDA_KERNEL_INCLUDE_DIR ${COHDA_ROOT}/ieee1609/kernel/include)
+set(COHDA_LIB1609_INCLUDE_DIR ${COHDA_ROOT}/ieee1609/app/lib1609/include)
+set(COHDA_LLC_INCLUDE_DIR ${COHDA_ROOT}/cohda/kernel/include/linux/cohda/llc)
+set(COHDA_INCLUDE_DIRS ${COHDA_KERNEL_INCLUDE_DIR} ${COHDA_LIB1609_INCLUDE_DIR} ${COHDA_LLC_INCLUDE_DIR})
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(Cohda "Cohda SDK not found" COHDA_ROOT)
+mark_as_advanced(COHDA_ROOT)
+if(COHDA_FOUND AND NOT TARGET Cohda::headers)
+    add_library(Cohda::headers UNKNOWN IMPORTED)
+    set_target_properties(Cohda::headers PROPERTIES
+        INTERFACE_INCLUDE_DIRECTORIES "${COHDA_INCLUDE_DIRS}")
+endif()