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FLASHING.md says how to take a newer version of the colleague's tree (copy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/vanetza/asn1/support/NULL_oer.c

@@ -0,0 +1,48 @@
+/*
+ * Copyright (c) 2017 Lev Walkin <vlm@lionet.info>.
+ * All rights reserved.
+ * Redistribution and modifications are permitted subject to BSD license.
+ */
+#include "asn_internal.h"
+#include "NULL.h"
+
+asn_dec_rval_t
+NULL_decode_oer(const asn_codec_ctx_t *opt_codec_ctx,
+                const asn_TYPE_descriptor_t *td,
+                const asn_oer_constraints_t *constraints, void **sptr,
+                const void *ptr, size_t size) {
+    asn_dec_rval_t rv = {RC_OK, 0};
+    (void)opt_codec_ctx;
+    (void)td;
+    (void)constraints;
+    (void)ptr;
+    (void)size;
+
+    if(!*sptr) {
+        *sptr = MALLOC(sizeof(NULL_t));
+        if(*sptr) {
+            *(NULL_t *)*sptr = 0;
+        } else {
+            ASN__DECODE_FAILED;
+        }
+    }
+
+    return rv;
+}
+
+asn_enc_rval_t
+NULL_encode_oer(const asn_TYPE_descriptor_t *td,
+                const asn_oer_constraints_t *constraints, const void *sptr,
+                asn_app_consume_bytes_f *cb, void *app_key) {
+    asn_enc_rval_t er = {0,0,0};
+
+    (void)td;
+    (void)sptr;
+    (void)constraints;
+    (void)cb;
+    (void)app_key;
+
+    er.encoded = 0;  /* Encoding in 0 bytes. */
+
+    ASN__ENCODED_OK(er);
+}