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FLASHING.md says how to take a newer version of the colleague's tree (copy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

obu-firmware/NOTES.md

@@ -3,7 +3,7 @@
 ## Since 2026-09-23: signed ITS on vanetza-idf
 
 This firmware is a port of the colleague's standalone ESP32-C5 VRU station
-(`microbu-esp32c5/firmware`, its own git repository kept beside this one and gitignored here).
+(`microbu-esp32c5/firmware`; a copy of that repository is kept in `microbu-esp32c5/`).
 From it: the vanetza-idf C-ITS stack (BTP, GeoNetworking, the TS 103 097 security entity with
 credentials in NVS), the station-link protocol v1 (`link_protocol.*`, `link_service.*`) over the
 native USB port (`serial_link.*`, frame type 0x10 in the same 0xAA55 framing as before) and over BLE