Place MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH in alphabetical order

The automatic merge of include/mbedtls/config.h had placed
MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH before
MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT.
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index d0d6bd9..d5502a9 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -1834,13 +1834,6 @@
 #define MBEDTLS_SSL_TRUNCATED_HMAC
 
 /**
- * \def MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH
- *
- * Enable modifying the maximum I/O buffer size.
- */
-//#define MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH
-
-/**
  * \def MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT
  *
  * Fallback to old (pre-2.7), non-conforming implementation of the truncated
@@ -1865,6 +1858,13 @@
 //#define MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT
 
 /**
+ * \def MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH
+ *
+ * Enable modifying the maximum I/O buffer size.
+ */
+//#define MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH
+
+/**
  * \def MBEDTLS_THREADING_ALT
  *
  * Provide your own alternate threading implementation.