Merge remote-tracking branch 'hanno/iotssl-1341-optional-certificate-verification-needs-ca-chain-backport-2.1' into mbedtls-2.1

* hanno/iotssl-1341-optional-certificate-verification-needs-ca-chain-backport-2.1:
  Add tests for missing CA chains and bad curves.
  Fix implementation of VERIFY_OPTIONAL verification mode