Improve failsafe mode

The failsafe mode could be made more failsafe.

(Parent ticket.)

Resources

• In live-build 3.x, one can customize failsafe options with --bootappend-live-failsafe.

Subtasks

• #5391 (closed)

Related issues

• Related to #6557 (closed)
• Related to #5691

Original created by @tails on 5570 (Redmine)