avb_add_hash_footer: fix rollback_index format

The --rollback_index value is interpreted as base 10 by default; fix the
avb_add_hash_footer rule to print values in base 10 rather than hex.

This doesn't affect any current build targets since all usages have
rollback_index < 10 at the moment so base 10 and hex are identical.

Test: a rollback_index of 12 failed build previously, now succeeds
Change-Id: Ib6cafacca02f8792a0bf3a1e733fc89ee16adebb