Adding NVRAM entries manually to support secureboot

[SoerenBusse]

The grub efi file signed by debian has a hardcoded path pointing to \EFI\debian\grub.cfg, so we need to install grub to this location. Then we would have two entries named "debian" in our boot menu and don't which one is for which disk. To overcome this issue we manually add the correctly labled entries using efibootmgr to our NVRAM.

This will fix issue: https://github.com/hn/debian-buster-zfs-root/issues/3