Backport 2.1:Add configuration file in md.h

include `*config.h*` in md.h as MACROS in the header file get ignored.
Fix for #1001.
diff --git a/include/mbedtls/md.h b/include/mbedtls/md.h
index b902355..00b1e3a 100644
--- a/include/mbedtls/md.h
+++ b/include/mbedtls/md.h
@@ -27,6 +27,12 @@
 
 #include <stddef.h>
 
+#if !defined(MBEDTLS_CONFIG_FILE)
+#include "config.h"
+#else
+#include MBEDTLS_CONFIG_FILE
+#endif
+
 #define MBEDTLS_ERR_MD_FEATURE_UNAVAILABLE                -0x5080  /**< The selected feature is not available. */
 #define MBEDTLS_ERR_MD_BAD_INPUT_DATA                     -0x5100  /**< Bad input parameters to function. */
 #define MBEDTLS_ERR_MD_ALLOC_FAILED                       -0x5180  /**< Failed to allocate memory. */