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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,52 @@
+/*
+ * 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 "asn_codecs_prim.h"
+#include "NULL.h"
+
+asn_enc_rval_t
+NULL_encode_xer(const asn_TYPE_descriptor_t *td, const void *sptr, int ilevel,
+                enum xer_encoder_flags_e flags, asn_app_consume_bytes_f *cb,
+                void *app_key) {
+    asn_enc_rval_t er = {0,0,0};
+
+    (void)td;
+    (void)sptr;
+    (void)ilevel;
+    (void)flags;
+    (void)cb;
+    (void)app_key;
+
+    /* XMLNullValue is empty */
+    er.encoded = 0;
+    ASN__ENCODED_OK(er);
+}
+
+
+static enum xer_pbd_rval
+NULL__xer_body_decode(const asn_TYPE_descriptor_t *td, void *sptr,
+                      const void *chunk_buf, size_t chunk_size) {
+    (void)td;
+    (void)sptr;
+    (void)chunk_buf;  /* Going to be empty according to the rules below. */
+
+    /*
+     * There must be no content in self-terminating <NULL/> tag.
+     */
+    if(chunk_size)
+        return XPBD_BROKEN_ENCODING;
+    else
+        return XPBD_BODY_CONSUMED;
+}
+
+asn_dec_rval_t
+NULL_decode_xer(const asn_codec_ctx_t *opt_codec_ctx,
+                const asn_TYPE_descriptor_t *td, void **sptr,
+                const char *opt_mname, const void *buf_ptr, size_t size) {
+    return xer_decode_primitive(opt_codec_ctx, td,
+        sptr, sizeof(NULL_t), opt_mname, buf_ptr, size,
+        NULL__xer_body_decode);
+}