util/exynos: Port *_cksum.py to python3

BUG=chromium:1023662
TEST=1. Create a tiny file `in.txt` as input
     2. Run `fixed_cksum.py in.txt out.txt 20` with py2 and py3 version,
     the output is the same
     3. Run `variable_cksum.py in.txt out.txt` with py2 and py3 version,
     the output is the same

Signed-off-by: Yilin Yang <kerker@google.com>
Change-Id: I9428269dfb826a3a95fffef9ea3f7c1a7107ef84
Reviewed-on: https://review.coreboot.org/c/coreboot/+/45460
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>

util/exynos/variable_cksum.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python2
+#!/usr/bin/env python3
 #
 # SPDX-License-Identifier: BSD-3-Clause
 
@@ -27,7 +27,7 @@ def main(argv):
     data = in_file.read()
     header = struct.pack(header_format,
                          struct.calcsize(header_format) + len(data),
-                         sum(map(ord, data)),
+                         sum(data),
                          0, 0)
     out_file.write(header + data)