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FLASHING.md says how to take a newer version of the colleague's tree (copy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/ecdsa256.hpp

@@ -0,0 +1,77 @@
+#ifndef ECDSA256_HPP_IOXLJFVZ
+#define ECDSA256_HPP_IOXLJFVZ
+
+#include <array>
+#include <cstdint>
+#include <functional>
+
+namespace vanetza
+{
+namespace security
+{
+
+// forward declaration
+struct Uncompressed;
+
+// do not use ecdsa256 for new v3 code, only for legacy v2 security!
+namespace ecdsa256
+{
+
+constexpr unsigned digest_octets = 32;
+
+struct PublicKey
+{
+    std::array<uint8_t, digest_octets> x;
+    std::array<uint8_t, digest_octets> y;
+};
+
+bool operator==(const PublicKey& lhs, const PublicKey& rhs);
+bool operator!=(const PublicKey& lhs, const PublicKey& rhs);
+
+
+struct PrivateKey
+{
+    std::array<uint8_t, digest_octets> key;
+};
+
+bool operator==(const PrivateKey& lhs, const PrivateKey& rhs);
+bool operator!=(const PrivateKey& lhs, const PrivateKey& rhs);
+
+
+struct KeyPair
+{
+    PrivateKey private_key;
+    PublicKey public_key;
+};
+
+/**
+ * Create generic public key from uncompressed ECC point
+ * \param unc Uncompressed ECC point
+ * \return public key
+ */
+PublicKey create_public_key(const Uncompressed&);
+
+} // namespace ecdsa256
+} // namespace security
+} // namespace vanetza
+
+
+namespace std
+{
+
+template<>
+struct hash<vanetza::security::ecdsa256::PublicKey>
+{
+    size_t operator()(const vanetza::security::ecdsa256::PublicKey&) const;
+};
+
+template<>
+struct hash<vanetza::security::ecdsa256::PrivateKey>
+{
+    size_t operator()(const vanetza::security::ecdsa256::PrivateKey&) const;
+};
+
+} // namespace std
+
+#endif /* ECDSA256_HPP_IOXLJFVZ */
+