Keep vanetza-idf in obu-firmware, so a plain clone builds the firmware

obu-firmware builds against the vanetza-idf C-ITS library, which until now
came from the colleague's microbu-esp32c5 tree beside the repository and was
not tracked here, so a clone of this repository could not build the firmware
it ships. The library alone is now part of obu-firmware, as
obu-firmware/external/vanetza-idf: their external/vanetza-idf at commit
cf4b99f, unchanged (9775 files; see its PROVENANCE.md). CMake takes it from
there by default; -DVANETZA_IDF_DIR still points the build elsewhere.

The rest of the colleague's tree (their own VAM firmware, PKI tooling,
station-link Python tools, the V2X2MAP bridge) stays out of this repository
and gitignored; nothing is pushed to their repository. NOTES.md, docs/06,
TODO.md and the pcap verifier's usage line point at the new location.

obu-firmware/external/vanetza-idf/vanetza/security/certificate_validity.hpp

@@ -0,0 +1,66 @@
+#ifndef BC8469A6_CC39_4826_A95E_DE639D68863B
+#define BC8469A6_CC39_4826_A95E_DE639D68863B
+
+#include <boost/optional/optional.hpp>
+
+namespace vanetza
+{
+namespace security
+{
+
+enum class CertificateInvalidReason
+{
+    Broken_Time_Period,
+    Off_Time_Period,
+    Unknown_Signer,
+    Missing_Signature,
+    Missing_Public_Key,
+    Invalid_Signer,
+    Invalid_Name,
+    Excessive_Chain_Length,
+    Off_Region,
+    Inconsistent_With_Signer,
+    Insufficient_ITS_AID,
+    Missing_Subject_Assurance,
+};
+
+class CertificateValidity
+{
+public:
+    CertificateValidity() = default;
+
+    /**
+     * Create CertificateValidity signalling an invalid certificate
+     * \param reason Reason for invalidity
+     */
+    CertificateValidity(CertificateInvalidReason reason) : m_reason(reason) {}
+
+    /**
+     * \brief Create CertificateValidity signalling a valid certificate
+     * This method is equivalent to default construction but should be more expressive.
+     * \return validity
+     */
+    static CertificateValidity valid() { return CertificateValidity(); }
+
+    /**
+     * Check if status corresponds to a valid certificate
+     * \return true if certificate is valid
+     */
+    operator bool() const { return !m_reason; }
+
+    /**
+     * \brief Get reason for certificate invalidity
+     * This call is only safe if reason is available, i.e. check validity before!
+     *
+     * \return reason
+     */
+    CertificateInvalidReason reason() const { return *m_reason; }
+
+private:
+    boost::optional<CertificateInvalidReason> m_reason;
+};
+    
+} // namespace security
+} // namespace vanetza
+
+#endif /* BC8469A6_CC39_4826_A95E_DE639D68863B */