BoringSSL: Map OpenSSL SUITEB192 cipher into appropriate sigalgs

BoringSSL: Map OpenSSL SUITEB192 cipher into appropriate sigalgs

BoringSSL removed the special OpenSSL cipher suite value "SUITEB192", so
need to map that to the explicit ciphersuite
(ECDHE-ECDSA-AES256-GCM-SHA384), curve (P-384), and sigalg
(SSL_SIGN_ECDSA_SECP384R1_SHA384) to allow 192-bit level Suite B with
ECDSA to be used.

This commit takes care of the sigalg configuration using the relatively
recent SSL_CTX_set_verify_algorithm_prefs() addition from April 2017.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>