API changes
   * The experimental functions psa_generate_key_ext() and
     psa_key_derivation_output_key_ext() have been replaced by
     psa_generate_key_custom() and psa_key_derivation_output_key_custom().
     They have almost exactly the same interface, but the variable-length
     data is passed in a separate parameter instead of a flexible array
     member. This resolves a build failure under C++ compilers that do not
     support flexible array members (a C99 feature not adopted by C++).
     Fixes #9020.