e2fsdroid: create incremental images

Add an option to read a base_fs file and allocate the blocks according
to the mapping provided by the file.

Test: 1/ Create a normal image and an incremental one.
         Compare the number of blocks that have changed.
      2/ Create an image.
         Create an incremantal image.
         The basefs file and the block_list file are the same.

Change-Id: Ie000ca48cf000d95e7a45a9752699abfc7484b6c
8 files changed