Keep vanetza-idf in obu-firmware, so a plain clone builds the firmware

obu-firmware builds against the vanetza-idf C-ITS library, which until now
came from the colleague's microbu-esp32c5 tree beside the repository and was
not tracked here, so a clone of this repository could not build the firmware
it ships. The library alone is now part of obu-firmware, as
obu-firmware/external/vanetza-idf: their external/vanetza-idf at commit
cf4b99f, unchanged (9775 files; see its PROVENANCE.md). CMake takes it from
there by default; -DVANETZA_IDF_DIR still points the build elsewhere.

The rest of the colleague's tree (their own VAM firmware, PKI tooling,
station-link Python tools, the V2X2MAP bridge) stays out of this repository
and gitignored; nothing is pushed to their repository. NOTES.md, docs/06,
TODO.md and the pcap verifier's usage line point at the new location.

obu-firmware/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);
+}