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
diff --git a/core/config.mk b/core/config.mk
index 196f07c..c747fd5 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -722,6 +722,7 @@
 BUILD_SUPER_IMAGE := $(BOARD_CUSTOM_BUILD_SUPER_IMAGE)
 endif
 IMG_FROM_TARGET_FILES := $(HOST_OUT_EXECUTABLES)/img_from_target_files$(HOST_EXECUTABLE_SUFFIX)
+UNPACK_BOOTIMG := $(HOST_OUT_EXECUTABLES)/unpack_bootimg
 MAKE_RECOVERY_PATCH := $(HOST_OUT_EXECUTABLES)/make_recovery_patch$(HOST_EXECUTABLE_SUFFIX)
 OTA_FROM_TARGET_FILES := $(HOST_OUT_EXECUTABLES)/ota_from_target_files$(HOST_EXECUTABLE_SUFFIX)
 OTA_FROM_RAW_IMG := $(HOST_OUT_EXECUTABLES)/ota_from_raw_img$(HOST_EXECUTABLE_SUFFIX)