Allow compile-time configuration of DTLS badmac limit

Introduces MBEDTLS_SSL_CONF_BADMAC_LIMIT to fix the maximum
number of records with bad MAC tolerated in DTLS at compile-time.

Impact on code-size:

|  | GCC | ARMC5 | ARMC6 |
| --- | --- | --- | --- |
| `libmbedtls.a` before  | 23511 | 24049 | 27903 |
| `libmbedtls.a` after | 23487 | 24025 | 27885 |
| gain in Bytes | 24 | 24 | 18 |
diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index 092819e..49c6050 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -1085,6 +1085,23 @@
  * be fixed at compile time via one of MBEDTLS_SSL_SSL_CONF_XXX.
  */
 
+#if defined(MBEDTLS_SSL_DTLS_BADMAC_LIMIT)
+#if !defined(MBEDTLS_SSL_CONF_BADMAC_LIMIT)
+static inline unsigned int mbedtls_ssl_conf_get_badmac_limit(
+    mbedtls_ssl_config  const *conf )
+{
+    return( conf->badmac_limit );
+}
+#else /* !MBEDTLS_SSL_CONF_BADMAC_LIMIT */
+static inline unsigned int mbedtls_ssl_conf_get_badmac_limit(
+    mbedtls_ssl_config  const *conf )
+{
+    ((void) conf);
+    return( MBEDTLS_SSL_CONF_BADMAC_LIMIT );
+}
+#endif /* MBEDTLS_SSL_CONF_BADMAC_LIMIT */
+#endif /* MBEDTLS_SSL_DTLS_BADMAC_LIMIT */
+
 #if defined(MBEDTLS_SSL_DTLS_ANTI_REPLAY)
 #if !defined(MBEDTLS_SSL_CONF_ANTI_REPLAY)
 static inline unsigned int mbedtls_ssl_conf_get_anti_replay(