Enable MBEDTLS_AES_ROM_TABLES in config-no-entropy

Enable the MBEDTLS_AES_ROM_TABLES option in the
configs/config-no-entropy.h to place AES lookup tables in ROM. This
saves considerable RAM space, a resource that is very limited in small
devices that use this configuration.
diff --git a/configs/config-no-entropy.h b/configs/config-no-entropy.h
index 95f17d4..7375860 100644
--- a/configs/config-no-entropy.h
+++ b/configs/config-no-entropy.h
@@ -80,6 +80,9 @@
 #define MBEDTLS_X509_CRT_PARSE_C
 #define MBEDTLS_X509_CRL_PARSE_C
 
+/* Miscellaneous options */
+#define MBEDTLS_AES_ROM_TABLES
+
 #include "check_config.h"
 
 #endif /* MBEDTLS_CONFIG_H */