liblp: Allow packing partition data into super images.

This change allows partition data to be included in the sparse image for
the super partition. The data is specified as a map from partition name
to partition image file. The image file can either be a normal file or a
sparse image. If it is sparse, it is unpacked into a temporary file and
then re-packed into the super image.

Bug: 79173901
Test: (1) lpmake -S with a partition image flashes to walleye.
      (2) The image can be mounted if it contains a filesystem.
      (3) simg2img will successfully unpack the image.

Change-Id: I98e9af3bfec9863143c1b95a1712b1d435942542