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

tools/releasetools/add_img_to_target_files.py

@@ -53,6 +53,7 @@ if sys.hexversion < 0x02070000:
 
 import datetime
 import errno
+import hashlib
 import os
 import shlex
 import shutil
@@ -546,6 +547,17 @@ def AddImagesToTargetFiles(filename):
 
   has_recovery = (OPTIONS.info_dict.get("no_recovery") != "true")
 
+  if OPTIONS.info_dict.get("avb_enable") == "true":
+    fp = None
+    if "build.prop" in OPTIONS.info_dict:
+      build_prop = OPTIONS.info_dict["build.prop"]
+      if "ro.build.fingerprint" in build_prop:
+        fp = build_prop["ro.build.fingerprint"]
+      elif "ro.build.thumbprint" in build_prop:
+        fp = build_prop["ro.build.thumbprint"]
+    if fp:
+      OPTIONS.info_dict["avb_salt"] = hashlib.sha256(fp).hexdigest()
+
   def banner(s):
     print("\n\n++++ " + s + " ++++\n\n")