Recovery mode

[o8vm]

Is your feature request related to a problem? Please describe.
Currently, Krabs does not have a way to recover from a boot failure, so it needs a reasonable recovery option that can be also used in a cloud environment like AWS EC2.

Describe the solution you'd like
For implementing Recovery Option, we are going to use a what should be called a startup success marker. Krabs uses this special marker by embedding it in the free space on the disk.

Then the following mechanism is used to detect startup failures and implement the fall back option.

1. After the OS start up succeeded, OS have to embed the success marker into disk with a service (like systemd services, etc).
2. If this mark is not present at the next time booting, bootloader judges that the OS failed to boot last time and tries to boot with another one.
3. Bootloader erase this marker before processing from the bootloader to the kernel.

This would make it possible to implement a simple recovery mode in Krabs that can be recovered by simply rebooting in a cloud environment.