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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,58 @@
+#include <vanetza/security/hmac.hpp>
+#include <algorithm>
+#include <array>
+
+#if defined VANETZA_WITH_OPENSSL
+#include <openssl/hmac.h>
+#endif
+
+#if defined VANETZA_WITH_CRYPTOPP
+#include <cryptopp/hmac.h>
+#include <cryptopp/sha.h>
+#endif
+
+namespace vanetza
+{
+namespace security
+{
+
+#if defined VANETZA_WITH_OPENSSL
+KeyTag create_hmac_tag_openssl(const ByteBuffer& data, const HmacKey& hmacKey)
+{
+    KeyTag keyTag;
+    std::array<std::uint8_t, EVP_MAX_MD_SIZE> tag;
+    unsigned int len = 0;
+    HMAC(EVP_sha256(), hmacKey.data(), static_cast<int>(hmacKey.size()),
+         data.data(), data.size(), tag.data(), &len);
+    std::copy_n(tag.data(), keyTag.size(), keyTag.data());
+    return keyTag;
+}
+#endif
+
+#if defined VANETZA_WITH_CRYPTOPP
+KeyTag create_hmac_tag_cryptopp(const ByteBuffer& data, const HmacKey& hmacKey)
+{
+    KeyTag keyTag;
+    std::array<std::uint8_t, 32> tag;
+    CryptoPP::HMAC<CryptoPP::SHA256> mac(hmacKey.data(), hmacKey.size());
+    mac.Update(data.data(), data.size());
+    mac.Final(tag.data());
+    std::copy_n(tag.data(), keyTag.size(), keyTag.data());
+    return keyTag;
+}
+#endif
+
+KeyTag create_hmac_tag(const ByteBuffer& data, const HmacKey& hmacKey)
+{
+#if defined VANETZA_WITH_OPENSSL
+    return create_hmac_tag_openssl(data, hmacKey);
+#elif defined VANETZA_WITH_CRYPTOPP
+    return create_hmac_tag_cryptopp(data, hmacKey);
+#else
+#   warning "no HMAC implementation available"
+    return KeyTag {};
+#endif
+}
+
+} // namespace security
+} // namespace vanetza