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FLASHING.md says how to take a newer version of the colleague's tree (copy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/btp/tests/header.cpp

@@ -0,0 +1,39 @@
+#include <gtest/gtest.h>
+#include <vanetza/btp/header.hpp>
+#include <vanetza/common/serialization_buffer.hpp>
+
+using namespace vanetza;
+
+const ByteBuffer ref = { 0x38, 0x84, 0x01, 0x40 };
+
+TEST(BtpHeaderA, serialize) {
+    btp::HeaderA header;
+    header.destination_port = host_cast<uint16_t>(0x3884);
+    header.source_port = host_cast<uint16_t>(0x0140);
+    ByteBuffer buf;
+    serialize_into_buffer(header, buf);
+    EXPECT_EQ(ref, buf);
+}
+
+TEST(BtpHeaderA, deserialize) {
+    btp::HeaderA header;
+    deserialize_from_buffer(header, ref);
+    EXPECT_EQ(host_cast<uint16_t>(0x3884), header.destination_port);
+    EXPECT_EQ(host_cast<uint16_t>(0x0140), header.source_port);
+}
+
+TEST(BtpHeaderB, serialize) {
+    btp::HeaderB header;
+    header.destination_port = host_cast<uint16_t>(0x3884);
+    header.destination_port_info = host_cast<uint16_t>(0x0140);
+    ByteBuffer buf;
+    serialize_into_buffer(header, buf);
+    EXPECT_EQ(ref, buf);
+}
+
+TEST(BtpHeaderB, deserialize) {
+    btp::HeaderB header;
+    deserialize_from_buffer(header, ref);
+    EXPECT_EQ(host_cast<uint16_t>(0x3884), header.destination_port);
+    EXPECT_EQ(host_cast<uint16_t>(0x0140), header.destination_port_info);
+}