releasetools: Include descriptor of `vbmeta_system.img` in `vbmeta.img`.

Chained vbmeta images should be included into the top-level vbmeta.img
as chained partitions. It's done in the path of `m`, but not `m dist`.

Bug: 118115607
Test: `m dist` a target that uses chained vbmeta.
    a) Check that the `vbmeta.img` in target-img.zip contains the image
       descriptor of `vbmeta_system.img` (`avbtool info_image`).
    b) `avbtool verify_image --image vbmeta.img \
            --expected_chain_partition \
                vbmeta_system:1:/path/to/vbmeta_system/pubkey`
    c) `avbtool verify_image --image vbmeta_system.img`
Change-Id: I064e583b247c44b9b2f19355838550bb5dbb8f26