Requirement changes
   * Clarify and test the import/export behaviour of PSA key management APIs to
     adhere to the to-be-introduced clarification. Montgomery keys
     (such as Curve25519) should be imported/exported in masked form.

Bugfix
   * Update and test the PSA key management against Montgomery keys, since
     these need to be imported/exported in little-endian form. Added mirror
     function of mbedtls_ecp_read_key called mbedtls_ecp_write_key to retrieve
     a private key in the correct form.