Use deterministic salt for AVB footer of prebuilt boot img

When target specified a prebuilt boot.img, current build system will add
avb hash footer to it with a random salt. Use a deterministic salt
instead for more reproducible builds. To stay consistent with
non-prebuilt boot.img code path, we extract the kernel image from
prebuilt boot.img and uses sha256sum of kernel image as the salt.

Test: th
Bug: 293313353
Change-Id: I988999ddc4f18e0b8677b05a3165c847b6a11b52