Convert init of eval to async option

So to be done only once at startup and in the (unlikely)
cases that a relevant UCI parameter is changed, instead
of doing it at the beginning of each search.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/uci.cpp

@@ -106,7 +106,7 @@ void uci_loop() {
 
       else if (token == "eval")
       {
-          read_evaluation_uci_options(pos.side_to_move());
+          EvalRootColor = pos.side_to_move();
           cout << trace_evaluate(pos) << endl;
       }