x86: Move table csum into separate file

We need the checksum function without all the other table functionality
soon, so let's split it out into its own C file.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

lib/tables_csum.c

@@ -0,0 +1,20 @@
+/*
+ * Copyright (C) 2015, Bin Meng <bmeng.cn@gmail.com>
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+#include <common.h>
+#include <linux/ctype.h>
+
+u8 table_compute_checksum(void *v, int len)
+{
+	u8 *bytes = v;
+	u8 checksum = 0;
+	int i;
+
+	for (i = 0; i < len; i++)
+		checksum -= bytes[i];
+
+	return checksum;
+}