Use computed salt for AVB-signed images.

We want the generated images being identical for the same source files.
Currently the generated ext4 image (either from make_ext4fs or mke2fs)
is reproducible, but the AVB footer added by avbtool contain changes
because of the random salt being used.

This CL changes the avbtool invocation to specify "--salt <hexstring>"
(already supported by avbtool) to use reproducible salt that's computed
based on fingerprints (or thumbprints if applicable).

Bug: 67023482
Test: Regenerate images from the same source as follows:
  Use a target_files.zip from an AVB-enabled target.
    $ zip -d target_files.zip IMAGES/\*
    $ ./build/make/tools/releasetools/add_img_to_target_files.py \
          -v target_files.zip
  Repeat the above commands and compare the generated images.
Change-Id: Id9db17ae0132ca3a820b4be5a5ef06ca3fef71ed
(cherry picked from commit 8f05cca1)