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FLASHING.md says how to take a newer version of the colleague's tree (copy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/asn1/release2/TS103916v211/README.md

@@ -0,0 +1,8 @@
+The TS 103916 v2.1.1 POIM ASN1 files are copied from its [ETSI repository](https://forge.etsi.org/rep/ITS/asn1/poim_ts103916/-/tree/v2.1.1).
+The submodule TS 103916 v2.1.1 PA (Parking Availability) ASN1 files are copied instead from [ETSI repository](https://forge.etsi.org/rep/ITS/asn1/pa_ts103916/-/tree/release2?ref_type=heads), due to a missing bug fix.
+
+Finally, the current ASN1 definition is manually patched to:
+- Import GeoPosition field from CDD 2.2.1, instead of redefining it
+- Define locally the EngineCharacteristics (renamed EngineCharacteristicsFromIso) field to avoid generation of all fields from ISO17573-3-1.asn 
+
+See [ETSI License](https://forge.etsi.org/rep/ITS/asn1/poim_ts103916/-/blob/v2.1.1/LICENSE) for copyright details.