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FLASHING.md says how to take a newer version of the colleague's tree (copy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