releasetools: Allow building AVB-enabled recovery.img.

Bug: 68224784
Test: Enable AVB on aosp_bullhead-userdebug. `m dist`. Note that the
      prebuilt vendor.img needs to be AVB-signed first.
Test: `m dist` with aosp_walleye-userdebug.
Change-Id: I18235e4d0dde6af71a96e46ec434480cc9a22bef