Add an lpunpack tool for extracting partitions from super.img.

Add an lpunpack tool for extracting partitions from super.img.

This tool performs the inverse of lpmake. It can extract partition
images out of a prebuilt super.img. There are a few caveats:

 - lpunpack does not support retrofit/split images.
 - lpunpack does not support sparse super images. They must be unsparsed
   before using with lpunpack.
 - Partition names will be suffixed on A/B devices, meaning,
   vendor_a.img will contain vendor.img and vendor_b.img will be
   0-length.

Note that lpunpack will store output files with holes for zero blocks,
as a space-saving measure.

Usage: lpunpack [options] SUPER_IMAGE [OUTPUT_DIR]

Bug: 131173010
Test: m superimage
      mm lpunpack
      lpunpack super.img
      lpunpack super.img out/
      lpunpack -p vendor_a super.img
      lpunpack -p unknown super.img
      sha1sum image with precursor image

Change-Id: Iad3f7362ae6ce767ddc7de2e8cfa7e4a47190e99
Merged-In: Iad3f7362ae6ce767ddc7de2e8cfa7e4a47190e99