Support building chained VBMeta images.

Bug: 112007947
Test: `m dist` with aosp_taimen-userdebug.
Test: Set up a target that uses chained VBMeta images of
      `vbmeta_mainline` and `vbmeta_vendor`. `m dist` and check the
      build log, as well as outputs from
      `avbtool info_image --image vbmeta.img`,
      `avbtool info_image --image vbmeta_mainline.img`,
      `avbtool info_image --image vbmeta_vendor.img`.
Change-Id: Ib1d4e97f583b65245703eae15d211adcd9e83741
diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index 8d51df6..ee2c6f4 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -408,7 +408,7 @@
     cmd.extend(["--key", key_path, "--algorithm", algorithm])
   avb_salt = OPTIONS.info_dict.get("avb_salt")
   # make_vbmeta_image doesn't like "--salt" (and it's not needed).
-  if avb_salt and partition != "vbmeta":
+  if avb_salt and not partition.startswith("vbmeta"):
     cmd.extend(["--salt", avb_salt])