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
diff --git a/filesystem/avb_add_hash_footer.go b/filesystem/avb_add_hash_footer.go
index ead579f..469f1fb 100644
--- a/filesystem/avb_add_hash_footer.go
+++ b/filesystem/avb_add_hash_footer.go
@@ -138,7 +138,7 @@
 		if rollbackIndex < 0 {
 			ctx.PropertyErrorf("rollback_index", "Rollback index must be non-negative")
 		}
-		cmd.Flag(fmt.Sprintf(" --rollback_index %x", rollbackIndex))
+		cmd.Flag(fmt.Sprintf(" --rollback_index %d", rollbackIndex))
 	}
 
 	cmd.FlagWithOutput("--image ", a.output)