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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,72 @@
+#ifndef FA909143_0CE7_4397_A42B_5CDA56AB4716
+#define FA909143_0CE7_4397_A42B_5CDA56AB4716
+
+#include <vanetza/security/ecc_point.hpp>
+#include <vanetza/security/signature.hpp>
+#include <cassert>
+
+namespace vanetza
+{
+namespace security
+{
+
+EcdsaSignatureFuture::EcdsaSignatureFuture(std::shared_future<EcdsaSignature> future,
+        EcdsaSignature placeholder) :
+    m_future(future), m_placeholder(std::move(placeholder))
+{
+    if (!m_future.valid()) {
+        throw std::invalid_argument("EcdsaSignature future has to be valid");
+    }
+}
+
+const EcdsaSignature& EcdsaSignatureFuture::get() const
+{
+    assert(m_future.valid());
+    const EcdsaSignature& signature = m_future.get();
+    assert(signature.s.size() == m_placeholder.s.size());
+    assert(signature.R.which() == m_placeholder.R.which());
+    assert(get_length(signature.R) == get_length(m_placeholder.R));
+    return signature;
+}
+
+std::size_t EcdsaSignatureFuture::size() const
+{
+    return get_length(m_placeholder.R) + m_placeholder.s.size();
+}
+
+ByteBuffer extract_signature_buffer(const SomeEcdsaSignature& sig)
+{
+    struct extraction_visitor : public boost::static_visitor<>
+    {
+        void operator()(const EcdsaSignature& sig)
+        {
+            m_buffer = convert_for_signing(sig.R);
+            m_buffer.insert(m_buffer.end(), sig.s.begin(), sig.s.end());
+        }
+
+        void operator()(const EcdsaSignatureFuture& sig_future)
+        {
+            const EcdsaSignature& sig = sig_future.get();
+            (*this)(sig);
+        }
+
+        ByteBuffer m_buffer;
+    };
+
+    extraction_visitor visitor;
+    boost::apply_visitor(visitor, sig);
+
+    return visitor.m_buffer;
+}
+
+ByteBuffer extract_signature_buffer(const Signature& sig)
+{
+    ByteBuffer buffer = sig.r;
+    buffer.insert(buffer.end(), sig.s.begin(), sig.s.end());
+    return buffer;
+}
+
+} // namespace security
+} // namespace vanetza
+
+#endif /* FA909143_0CE7_4397_A42B_5CDA56AB4716 */