Add early data support preparatory work

Add MBEDTLS_SSL_EARLY_DATA configuration option
Define early_data_enabled field in mbedtls_ssl_config
Add function mbedtls_ssl_conf_early_data

Signed-off-by: Xiaokang Qian <xiaokang.qian@arm.com>
diff --git a/include/mbedtls/mbedtls_config.h b/include/mbedtls/mbedtls_config.h
index 9588ca4..8e45726 100644
--- a/include/mbedtls/mbedtls_config.h
+++ b/include/mbedtls/mbedtls_config.h
@@ -1633,6 +1633,14 @@
 #define MBEDTLS_SSL_TLS1_3_DEFAULT_NEW_SESSION_TICKETS 1
 
 /**
+* \def MBEDTLS_SSL_EARLY_DATA
+*
+* Allows to add functionality for TLS/DTLS 1.3 early data.
+*
+*/
+//#define MBEDTLS_SSL_EARLY_DATA
+
+/**
  * \def MBEDTLS_SSL_PROTO_DTLS
  *
  * Enable support for DTLS (all available versions).