Give a production-sounding name to the p256m option

Now that p256-m is officially a production feature and not just an example,
give it a more suitable name.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/mbedtls/mbedtls_config.h b/include/mbedtls/mbedtls_config.h
index ce8395e..52fab1f 100644
--- a/include/mbedtls/mbedtls_config.h
+++ b/include/mbedtls/mbedtls_config.h
@@ -883,7 +883,7 @@
  *   #PSA_WANT_KEY_TYPE_ECC_KEY_PAIR_EXPORT and/or
  *   #PSA_WANT_KEY_TYPE_ECC_KEY_PAIR_GENERATE as needed.
  */
-//#define MBEDTLS_P256M_EXAMPLE_DRIVER_ENABLED
+//#define MBEDTLS_PSA_P256M_DRIVER_ENABLED
 
 /**
  * \def MBEDTLS_ECDSA_DETERMINISTIC