set up the salt for partitions without build.prop

BUG:242954151

Test: make target-files-package, then check the two sha1 are same:
      1. sha1 of IMAGES/boot.img
      2. expected sha1 of boot in VENDOR/bin/install-recovery.sh

Test: make target-files-package; unzip target_files.zip IMAGES/\*`;
      `zip -d arget_files.zip IMAGES/\*`
      `add_img_to_target_files -a target_files.zip`.
      Verify that the rebuilt images are identical to the deleted ones.

Change-Id: I5906ac1696b6403674fd8330b1569aed53510945