Implement emergency shell

[anatol]

There are situations when root filesystem mount failed (incorrect boot params, bug in booster, timeout issue with networks, ...). Currently booster either panics or hangs forever.

But it would be great if a user is able to inspect the machine and check what is going on. Or mount/switch to the root fs manually.

We need a way to add shell + utils (such as busybox optimized for size) and then switch to this emergency shell if something is going wrong.

We also need to preserve debug log and store it under /run/booster/debug.log so users will be able to copy it to a USB storage for further investigation.