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/config.h b/include/mbedtls/config.h
index 1ff34dc..ee292c5 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -3450,8 +3450,9 @@
  * \{
  */
 
-/* DTLS Anti replay */
+/* DTLS-specific settings */
 //#define MBEDTLS_SSL_CONF_ANTI_REPLAY MBEDTLS_SSL_ANTI_REPLAY_ENABLED
+//#define MBEDTLS_SSL_CONF_BADMAC_LIMIT 0
 
 /* ExtendedMasterSecret extension
  * The following two options must be set/unset simultaneously. */