Removals
   * Remove config option MBEDTLS_TLS_DEFAULT_ALLOW_SHA1_IN_CERTIFICATES,
     which allowed SHA-1 in the default TLS configuration for certificate
     signing. It was intended to facilitate the transition in environments
     with SHA-1 certificates. SHA-1 is considered a weak message digest and
     its use constitutes a security risk.

Changes
   * Set config option MBEDTLS_TLS_DEFAULT_ALLOW_SHA1_IN_KEY_EXCHANGE to be
     disabled by default.
