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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,80 @@
+#ifndef OSI_LAYER_HPP_C4VTEZJP
+#define OSI_LAYER_HPP_C4VTEZJP
+
+#include <boost/range/iterator_range.hpp>
+#include <array>
+#include <cstdint>
+
+namespace vanetza
+{
+
+enum class OsiLayer : uint8_t
+{
+    Physical = 1,
+    Link = 2,
+    Network = 3,
+    Transport = 4,
+    Session = 5,
+    Presentation = 6,
+    Application = 7
+};
+
+constexpr OsiLayer min_osi_layer() { return OsiLayer::Physical; }
+constexpr OsiLayer max_osi_layer() { return OsiLayer::Application; }
+
+constexpr std::array<OsiLayer, 7> osi_layers {{
+            OsiLayer::Physical,
+            OsiLayer::Link,
+            OsiLayer::Network,
+            OsiLayer::Transport,
+            OsiLayer::Session,
+            OsiLayer::Presentation,
+            OsiLayer::Application
+}};
+
+/**
+ * Calculate distance between layers
+ * \param from start counting at this layer
+ * \param to stop counting here
+ * \return 0 if equal layers, positive if "to" is a higher layer, negative if "to" is below "from"
+ */
+constexpr int distance(OsiLayer from, OsiLayer to)
+{
+    return static_cast<int>(to) - static_cast<int>(from);
+}
+
+constexpr std::size_t num_osi_layers(OsiLayer from, OsiLayer to)
+{
+    return (from <= to ? distance(from, to) + 1 : 0);
+}
+
+template<OsiLayer FROM, OsiLayer TO>
+std::array<OsiLayer, num_osi_layers(FROM, TO)> osi_layer_range()
+{
+    static_assert(FROM <= TO, "FROM layer is above TO layer");
+    typedef typename std::underlying_type<OsiLayer>::type num_type;
+
+    num_type num = static_cast<num_type>(FROM);
+    std::array<OsiLayer, num_osi_layers(FROM, TO)> layers;
+    for (auto& layer : layers) {
+        layer = static_cast<OsiLayer>(num++);
+    }
+    return layers;
+}
+
+inline boost::iterator_range<decltype(osi_layers)::const_iterator>
+osi_layer_range(OsiLayer from, OsiLayer to)
+{
+    if (from <= to) {
+        auto begin = osi_layers.cbegin() + distance(min_osi_layer(), from);
+        auto end = osi_layers.cend() - distance(to, max_osi_layer());
+        return boost::make_iterator_range(begin, end);
+    } else {
+        return boost::make_iterator_range(osi_layers.cend(), osi_layers.cend());
+    }
+}
+
+} // namespace vanetza
+
+#endif /* OSI_LAYER_HPP_C4VTEZJP */
+