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