AOSP: e2fsdroid: create incremental images

AOSP: 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
From AOSP commit: 16babe7b79c4c9b6d75d60e30c04a8e24278e4fa

Signed-off-by: Theodore Ts'o <tytso@mit.edu>