Add md/shaXXX_clone() API

Will be used in the SSL/TLS modules
diff --git a/library/md2.c b/library/md2.c
index 5cd024a..d1fea12 100644
--- a/library/md2.c
+++ b/library/md2.c
@@ -97,6 +97,12 @@
     mbedtls_zeroize( ctx, sizeof( mbedtls_md2_context ) );
 }
 
+void mbedtls_md2_clone( mbedtls_md2_context *dst,
+                        const mbedtls_md2_context *src )
+{
+    *dst = *src;
+}
+
 /*
  * MD2 context setup
  */