fastboot: sparse: resync common/image-sparse.c (part 1)

This file originally came from upstream code.

While retaining the storage abstraction feature, this is the first
set of the changes required to resync with the
  cmd_flash_mmc_sparse_img()
in the file
  aboot.c
from
  https://us.codeaurora.org/cgit/quic/la/kernel/lk/plain/app/aboot/aboot.c?h=LE.BR.1.2.1



Signed-off-by: Steve Rae <srae@broadcom.com>