Backport 1.3:Add configuration file in md.h

include *`config.h`* in md.h as MACROS in the header file get ignored.Backport to
Backport of #1055 to mbedtls-1.3
diff --git a/include/polarssl/md.h b/include/polarssl/md.h
index fc7482a..e395870 100644
--- a/include/polarssl/md.h
+++ b/include/polarssl/md.h
@@ -33,6 +33,12 @@
 #define inline __inline
 #endif
 
+#if !defined(POLARSSL_CONFIG_FILE)
+#include "config.h"
+#else
+#include POLARSSL_CONFIG_FILE
+#endif
+
 #define POLARSSL_ERR_MD_FEATURE_UNAVAILABLE                -0x5080  /**< The selected feature is not available. */
 #define POLARSSL_ERR_MD_BAD_INPUT_DATA                     -0x5100  /**< Bad input parameters to function. */
 #define POLARSSL_ERR_MD_ALLOC_FAILED                       -0x5180  /**< Failed to allocate memory. */