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FLASHING.md says how to take a newer version of the colleague's tree (copy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/examples/esp_idf_test/main/main.cpp

@@ -0,0 +1,30 @@
+#include <cstdio>
+#include <esp_task_wdt.h>
+#include <nvs_flash.h>
+#include <freertos/FreeRTOS.h>
+#include <freertos/task.h>
+int vidf_test_main();
+void run_hil_server();
+extern "C" void app_main() {
+    // The ESP32-C5 ROM UART0 clock repair for warm resets is applied by the component
+    // itself (ports/esp_idf/src/esp32c5_rom_uart_clock.c) before app_main runs.
+    std::puts("vanetza-idf component tests beginning");
+    // NVS for the credential store tests (the application's job, credentials.hpp)
+    esp_err_t nvs = nvs_flash_init();
+    if (nvs == ESP_ERR_NVS_NO_FREE_PAGES || nvs == ESP_ERR_NVS_NEW_VERSION_FOUND) {
+        ESP_ERROR_CHECK(nvs_flash_erase());
+        nvs = nvs_flash_init();
+    }
+    ESP_ERROR_CHECK(nvs);
+    // The security tests run about a minute of software ECDSA without yielding; give the
+    // idle-task watchdog that long instead of interleaving its reports with the output.
+    esp_task_wdt_config_t watchdog = {};
+    watchdog.timeout_ms = 300000;
+    watchdog.idle_core_mask = (1 << portNUM_PROCESSORS) - 1;
+    watchdog.trigger_panic = false;
+    esp_task_wdt_reconfigure(&watchdog);
+    const int result = vidf_test_main();
+    std::printf("VIDF_TEST_RESULT=%d\n", result);
+    if (result == 0) run_hil_server();
+    for (;;) vTaskDelay(pdMS_TO_TICKS(1000));
+}