Allow config'n of incl of CertificateReq CA list Y/N at compile-time

Introduces MBEDTLS_SSL_CONF_CERT_REQ_CA_LIST which allows to configure
at compile-time whether a CA list should be included in the
CertificateRequest message sent by the server.

Impact on code-size:

|  | GCC 8.2.1 | ARMC5 5.06 | ARMC6 6.12 |
| --- | --- | --- | --- |
| `libmbedtls.a` before  | 23131 | 23805 | 26673 |
| `libmbedtls.a` after | 23099 | 23781 | 26639 |
| gain in Bytes | 32 | 24 | 34 |
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 86759e5..7c5cadc 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -1106,8 +1106,10 @@
     unsigned int fallback : 1;      /*!< is this a fallback?                */
 #endif
 #if defined(MBEDTLS_SSL_SRV_C)
+#if !defined(MBEDTLS_SSL_CONF_CERT_REQ_CA_LIST)
     unsigned int cert_req_ca_list : 1;  /*!< enable sending CA list in
                                           Certificate Request messages?     */
+#endif /* !MBEDTLS_SSL_CONF_CERT_REQ_CA_LIST */
 #endif
 #if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)
 #if !defined(MBEDTLS_SSL_CONF_IGNORE_UNEXPECTED_CID)
@@ -2965,19 +2967,22 @@
 void mbedtls_ssl_conf_arc4_support( mbedtls_ssl_config *conf, char arc4 );
 #endif /* MBEDTLS_ARC4_C */
 
-#if defined(MBEDTLS_SSL_SRV_C)
+#if defined(MBEDTLS_SSL_SRV_C) && !defined(MBEDTLS_SSL_CONF_CERT_REQ_CA_LIST)
 /**
  * \brief          Whether to send a list of acceptable CAs in
  *                 CertificateRequest messages.
  *                 (Default: do send)
  *
+ * \note           On constrained systems, this options can also be configured
+ *                 at compile-time via MBEDTLS_SSL_CONF_CERT_REQ_CA_LIST.
+ *
  * \param conf     SSL configuration
  * \param cert_req_ca_list   MBEDTLS_SSL_CERT_REQ_CA_LIST_ENABLED or
  *                          MBEDTLS_SSL_CERT_REQ_CA_LIST_DISABLED
  */
 void mbedtls_ssl_conf_cert_req_ca_list( mbedtls_ssl_config *conf,
                                           char cert_req_ca_list );
-#endif /* MBEDTLS_SSL_SRV_C */
+#endif /* MBEDTLS_SSL_SRV_C && !MBEDTLS_SSL_CONF_CERT_REQ_CA_LIST */
 
 #if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
 /**