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/UseVanetza.cmake

@@ -0,0 +1,56 @@
+#
+# vanetza_optional_dependency(<package> [<version>] <option> <description>)
+#
+# Probe for <package> quietly, create a CMake option defaulting to whether it
+# was found, and require it if the option is enabled.
+#
+macro(vanetza_optional_dependency package)
+    if(${ARGC} EQUAL 4)
+        set(_vanetza_dep_version ${ARGV1})
+        set(_vanetza_dep_option ${ARGV2})
+        set(_vanetza_dep_description "${ARGV3}")
+    else()
+        set(_vanetza_dep_version)
+        set(_vanetza_dep_option ${ARGV1})
+        set(_vanetza_dep_description "${ARGV2}")
+    endif()
+    find_package(${package} ${_vanetza_dep_version} QUIET)
+    option(${_vanetza_dep_option} "${_vanetza_dep_description}" ${${package}_FOUND})
+    if(${_vanetza_dep_option})
+        find_package(${package} ${_vanetza_dep_version} REQUIRED)
+    endif()
+    unset(_vanetza_dep_version)
+    unset(_vanetza_dep_option)
+    unset(_vanetza_dep_description)
+endmacro()
+
+#
+# add_vanetza_component(<name> <sources...>)
+#
+function(add_vanetza_component name)
+    set(sources ${ARGN})
+
+    add_library(${name} ${sources})
+    add_library(Vanetza::${name} ALIAS ${name})
+    target_include_directories(${name} PUBLIC
+        $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}>
+        $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
+    set_target_properties(${name} PROPERTIES
+        OUTPUT_NAME vanetza_${name}
+        SOVERSION ${VANETZA_SOVERSION}
+        INSTALL_RPATH $ORIGIN)
+    target_compile_features(${name} PUBLIC cxx_std_14)
+    install(TARGETS ${name} EXPORT ${PROJECT_NAME} DESTINATION ${CMAKE_INSTALL_LIBDIR})
+    set_property(GLOBAL APPEND PROPERTY VANETZA_COMPONENTS ${name})
+endfunction()
+
+#
+# add_test_subdirectory(<directory>)
+#
+# Add subdirectory only when tests are enabled via BUILD_TESTS
+#
+function(add_test_subdirectory directory)
+  if(BUILD_TESTS)
+    add_subdirectory(${directory})
+  endif()
+endfunction(add_test_subdirectory)