Soong support for rollback index.

avbtool already supports --rollback_index. Allow soong support as well
to enable images to include it in their targets.

Test: Builds
Test: avbtool info_image on a target built using rule `avb_add_hash_footer`
Bug: 296830692
Change-Id: Id32f30d026b01172c5dadc0698938acb2c2c8e35