Add rule ChecksumFileSHA256

build/jam/FileRules

@@ -437,3 +437,10 @@ rule DownloadFile file : url : source
 	return $(file) ;
 }
 
+
+actions ChecksumFileSHA256
+{
+	$(HOST_SHA256) $(2) | sed -r 's,([^[:space:]]*).*,\1,' > $(1)
+		# The sed part is only necessary for sha256sum, but it doesn't harm for
+		# sha256 either.
+}