- certs.h is gone. Only contains test data, and was not used at all.
- config.h is renamed. Was seemingly not used, so can be removed.
- MBEDTLS_ERR_SSL_NO_USABLE_CIPHERSUITE is gone, instead
  MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE will be thrown.
- mbedtls_pk_parse_keyfile now needs to be passed a properly seeded
  RNG. Hence, move the call to after RNG seeding.
Signed-off-by: Timo Rothenpieler <timo@rothenpieler.org>