partition_tools: Add an "lpadd" tool for appending images to super.img.

partition_tools: Add an "lpadd" tool for appending images to super.img.

Syntax: lpadd [--readonly] SUPER_FILE PART_NAME GROUP_NAME [IMAGE_FILE]

The super file can be sparsed, unsparsed, or a "super_empty" image. The
source image can either be sparsed or unsparsed as well. The named
partition cannot already exist.

Usage is further described in README.md.

Bug: 144468779
Test: lpadd unsparsed super.img w/ sparsed partition
      lpadd unsparsed super.img w/ unsparsed partition
      lpadd sparsed super.img w/ sparsed partition
      lpadd sparsed super.img w/ unsparsed partition
      lpadd super_empty.img

Change-Id: I0cb5d4432c86689567f2a58a379be0f96b64a88d