Adds a menu selector to the grub boot options that launches
zfsbootmanger as a payload, a self-contained Linux system that knows how
to find other Linux kernels and initramfs images within ZFS filesystems.
When a suitable kernel and initramfs are identified (either through an
automatic process or direct user selection), ZFSBootMenu launches that
kernel using the kexec command.
Key Features
* Boot Environment Management: Allows switching between multiple OS
installations or versions (e.g., Ubuntu, Void Linux) stored as ZFS
datasets.
* Snapshot Integration: Enables booting from any ZFS snapshot before the
OS loads, ideal for recovery or testing.
* Full Disk Encryption Support: Prompts for encryption passphrases
during boot and unlocks encrypted ZFS pools automatically.
* UEFI Booting: Supports direct UEFI booting via efibootmgr or
third-party boot managers like rEFInd.
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