Add another option to reduce EC memory usage

Also document speed/memory trade-offs better.
diff --git a/include/polarssl/config.h b/include/polarssl/config.h
index d98bdb3..16643dc 100644
--- a/include/polarssl/config.h
+++ b/include/polarssl/config.h
@@ -1874,6 +1874,7 @@
 //
 #define POLARSSL_ECP_MAX_BITS             521 /**< Maximum bit size of groups */
 #define POLARSSL_ECP_WINDOW_SIZE            6 /**< Maximum window size used */
+#define POLARSSL_ECP_FIXED_POINT_OPTIM      1 /**< Enable fixed-point speed-up */
 
 // Entropy options
 //