Parse Android boot.img.

The Android boot.img contains kernel, ramdisk and optionally second
stage bootloader. Both kernel and ramdisk could be compressed by gzip.

This patch add a new boot img filesystem to parse boot.img to split
it into |File|s, and find the deflates to utilize puffin, which could
save us several MBs in delta payload size.

As a side effect, generating delta payload for boot partition is much
faster because it is now splitted into smaller files, and we can make
use of multithreading.

Bug: 110494725
Test: generate a delta payload
Change-Id: If41468b37a407fe1b7a70c2b61f632f7df8176d5