Remove dependency from SSL on PK internals

So far, with MBEDTLS_SSL_KEEP_PEER_CERTIFICATE disabled, the SSL module relied
on a undocumented feature of the PK module: that you can distinguish between
contexts that have been setup and context that haven't. This feature is going
to go away in the case of PK_SINGLE_TYPE, as we'll soon (as in: the next
commit does that) no longer be storing the (now two-valued) pk_info member.

Note even with this change, we could still distinguish if the context has been
set up by look if pk_ctx is NULL or not, but this is also going away in the
near future (a few more commits down the road), so not a good option either.

include/mbedtls/ssl_internal.h

@@ -382,9 +382,9 @@ struct mbedtls_ssl_sig_hash_set_t
  */
 struct mbedtls_ssl_handshake_params
 {
-    /*
-     * Handshake specific crypto variables
-     */
+#if !defined(MBEDTLS_SSL_KEEP_PEER_CERTIFICATE)
+    uint8_t got_peer_pubkey;            /*!< Did we store the peer's public key from its certificate? */
+#endif /* !MBEDTLS_SSL_KEEP_PEER_CERTIFICATE */
 #if defined(MBEDTLS_SSL_PROTO_DTLS)
     unsigned char verify_cookie_len;    /*!<  Cli: cookie length
                                               Srv: flag for sending a cookie */