Uses a per-partition fingerprint for building images and avb_salt.

This causes the output image files of a merged build to be identical
to the image files of the input partial builds, for each images in
PARTITIONS_WITH_CARE_MAP.

Test: python -m unittest test_common
Test: `m dist`; `unzip out/dist/target_files.zip IMAGES/\*`;
      `zip -d out/dist/target_files.zip IMAGES/\*`
      `add_img_to_target_files -a out/dist/target_files.zip`.
      Verify that the rebuilt images are identical to the deleted ones.
Test: Build a merged target (using merge_target_files.py). Verify that
      the partial target-files.zip IMAGES are identical to the merged
      target-files.zip IMAGES for PARTITIONS_WITH_CARE_MAP images.
Bug: 150405807
Change-Id: I5fdf5783c1aff9c14cf5408090389b1f65b69ca6