Assign shen-en as some random starting issues.

Chromium OS supports three ways to boot to kernel:

1. x86 legacy BIOS
   --> read MBR of each driver, and execute it
2. x86 EFI BIOS
   --> find EFI System Partition, and run bootx64.efi
3. Chrome OS Devices
   --> find GPT partitions with ChromeOS Kernel Type GUID
   --> should only have two kernels with this type
   --> boot to the recent/valid one

We're focusing only on the third way. And there are several reasons causing a boot failure in this way:
1. Invalid GPT table
2. Wrong GPT partitions:
   * Kernel partitions should be with ChromeOS Kernel Type GUID
   * Should have more than 1 kernel partitions
   * Priority/Success/Tries attributes should be correct.
3. dmverity error
4. firmware/kernel/driver error

It would be good if we can catch 1~3 errors automatically. However, the dmverity error is hard (or impossible?) to be tackled when running under that kernel. Since when we're reading that block, dmverity will block it and halt/reboot the machine.

So, we're focusing on the GPT-related errors. Several things we can do here:
1. Check GPT table is valid
2. Check there're more than one ChromeOS kernel partitions
3. Exactly one of the kernel partition is with successful=1. This is the active kernel partition.

In most cases, we're not changing active kernel, so we can do one more check:
4. Check the active kernel partition is with the highest priority

These checks can be implemented in 'shutdown' pytest, as a pre-shutdown check.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/factory/+/b0b7aee5e34a88b3c7a92734cfbe54a4f91d9131

commit b0b7aee5e34a88b3c7a92734cfbe54a4f91d9131
Author: Shen-En Shih <petershih@chromium.org>
Date: Fri Sep 22 05:36:24 2017

shutdown: Check partition layout before shutdown

Check if the partition layout looks good for the next boot
before shutdown or reboot.

BUG= chromium:717887 
TEST=make test
TEST=manually test

Change-Id: I82fdedb25bdf3a33b3e5d4bf1dc37b2e261588ee
Reviewed-on: https://chromium-review.googlesource.com/674585
Commit-Ready: Shen-En Shih <petershih@chromium.org>
Tested-by: Shen-En Shih <petershih@chromium.org>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>

[modify] https://crrev.com/b0b7aee5e34a88b3c7a92734cfbe54a4f91d9131/py/test/pytests/shutdown/shutdown.py
[modify] https://crrev.com/b0b7aee5e34a88b3c7a92734cfbe54a4f91d9131/py/utils/sys_utils.py