liblp: Split super.img for retrofit devices.

On retrofit devices, we need images that can be flashed in the
bootloader for the component "super"partitions. This change rewrites
SparseBuilder so that it generates one sparse image per block device,
and now handles partitions that span across multiple block devices.

A new API function has been added to write the set of sparse images to
an output folder.

Bug: 118887112
Test: manual test, flash split images
Change-Id: Iff56efdcb7bdfd5bc8dd7ff44e8234e091ac2346