Description: Use Python 3 for `mkbootimg`

Bug: https://android.googlesource.com/platform/system/core/+/c434cf82d526f07cd3dcebf53582eeddf12b0a74

--- a/mkbootimg/mkbootimg.py

+++ b/mkbootimg/mkbootimg.py

@@ -1,4 +1,4 @@

-#!/usr/bin/env python

+#!/usr/bin/python3

 # Copyright 2015, The Android Open Source Project

 #

 # Licensed under the Apache License, Version 2.0 (the "License");

--- a/mkbootimg/unpack_bootimg.py

+++ b/mkbootimg/unpack_bootimg.py

@@ -1,4 +1,4 @@

-#!/usr/bin/env python

+#!/usr/bin/python3

 # Copyright 2018, The Android Open Source Project

 #

 # Licensed under the Apache License, Version 2.0 (the "License");