Changes
   * Changed the default MBEDTLS_ECP_WINDOW_SIZE from 6 to 2.
     As tested in issue 6790, the correlation between this define and
     RSA decryption performance has changed lately due to security fixes.
     To fix the performance degradation when using default values the
     window was reduced from 6 to 2, a value that gives the best or close
     to best results when tested on Cortex-M4 and Intel i7.
