proxmox-boot-tool: handle legacy boot zfs installs

proxmox-boot-tool: handle legacy boot zfs installs

This patch adds support for booting non-uefi/legacy/bios-boot ZFS
installs, by using proxmox-boot-tool to copy the kernels to the ESP
and then generate a fitting grub config for booting from the vfat ESP:

* grub is installed onto the ESP and the MBR points to the ESP
* after copying/deleting the kernels proxmox-boot-tool bindmounts the
  ESP on /boot (inside the new mount namespace)
* grub-update then manages to generate a fitting config.

Some paths/sanity-checks needed adaptation (to differentiate between
EFI boot and not (based on the existence of /sys/firmware/efi)

The arguments for grub-install are taken from the pve-installer.

The approach is inspired by @avw in our community-forum [0].

[0] https://forum.proxmox.com/threads/zfs-error-no-such-device-error-unknown-filesystem-entering-rescue-mode.75122/post-374799

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>