Add server-side support for asynchronous signing.

The RSA key exchange needs decryption and is still unsupported.

Change-Id: I8c13b74e25a5424356afbe6e97b5f700a56de41f
Reviewed-on: https://boringssl-review.googlesource.com/5467
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/ssl_rsa.c b/ssl/ssl_rsa.c
index 7b6cd75..da69f8b 100644
--- a/ssl/ssl_rsa.c
+++ b/ssl/ssl_rsa.c
@@ -632,6 +632,10 @@
   ssl->cert->key_method = key_method;
 }
 
+int ssl_has_private_key(SSL *ssl) {
+  return ssl->cert->privatekey != NULL || ssl->cert->key_method != NULL;
+}
+
 int ssl_private_key_type(SSL *ssl) {
   if (ssl->cert->key_method != NULL) {
     return ssl->cert->key_method->type(ssl);