releasetools: Support verifying AVB signed images with chained partitions.

For example, verify a target_files.zip that has system AVB-signed as a
chained partition.

  $ build/make/tools/releasetools/validate_target_files.py \
      signed-target_files-4904652.zip \
      --verity_key verifiedboot_pub.pem \
      --avb_system_key_path system_pub.pem

Note that verifiedboot_pub.pem should be the key (either public or
private) to verify vbmeta.img, and 'system_pub.pem' should be the key
(either public or private) for the chained partition of system.

testdata/testkey.key is the private key converted from
testdata/testkey.pk8 for testing purpose (`openssl pkcs8 -in
testdata/testkey.pk8 -inform DER -out testdata/testkey.key -nocrypt`).

Bug: 63706333
Test: python -m unittest test_common
Test: python -m unittest test_add_img_to_target_files
Test: `m dist` on aosp_walleye-userdebug; Run validate_target_files.py
      on the generated target_files.zip.
Test: Set up walleye with chained system partition; `m dist`; Run
      validate_target_files.py on the generated target_files.zip.
Change-Id: I38517ab39baf8a5bc1a6062fab2fe229b68e897d

tools/releasetools/add_img_to_target_files.py

@@ -373,22 +373,9 @@ def AppendVBMetaArgsForPartition(cmd, partition, image):
   # Check if chain partition is used.
   key_path = OPTIONS.info_dict.get("avb_" + partition + "_key_path")
   if key_path:
-    # extract public key in AVB format to be included in vbmeta.img
-    avbtool = os.getenv('AVBTOOL') or OPTIONS.info_dict["avb_avbtool"]
-    pubkey_path = common.MakeTempFile(prefix="avb-", suffix=".pubkey")
-    proc = common.Run(
-        [avbtool, "extract_public_key", "--key", key_path, "--output",
-         pubkey_path],
-        stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
-    stdoutdata, _ = proc.communicate()
-    assert proc.returncode == 0, \
-        "Failed to extract pubkey for {}:\n{}".format(
-            partition, stdoutdata)
-
-    rollback_index_location = OPTIONS.info_dict[
-        "avb_" + partition + "_rollback_index_location"]
-    cmd.extend(["--chain_partition", "%s:%s:%s" % (
-        partition, rollback_index_location, pubkey_path)])
+    chained_partition_arg = common.GetAvbChainedPartitionArg(
+        partition, OPTIONS.info_dict)
+    cmd.extend(["--chain_partition", chained_partition_arg])
   else:
     cmd.extend(["--include_descriptors_from_image", image])