Implement hmac in the MD layer

include/mbedtls/md.h

@@ -78,6 +78,10 @@ typedef struct {
 
     /** Digest-specific context */
     void *md_ctx;
+
+    /** HMAC part of the context (WIP: fixed size) */
+    unsigned char ipad[128];
+    unsigned char opad[128];
 } md_context_t;
 
 #define MD_CONTEXT_T_INIT { \

include/mbedtls/md_wrap.h

@@ -54,6 +54,9 @@ struct _md_info_t {
     /** Output length of the digest function */
     int size;
 
+    /** Block length of the digest function */
+    int block_size;
+
     /** Digest initialisation function */
     void (*starts_func)( void *ctx );