Since GRUB installation was migrated to vyos-1x from vyatta-cfg-system (T4516), the GRUB setup has hardcoded references to x86-64, undoing the arm64 support that was previously in vyatta-cfg-system (T4822).

This task adds arm64 support back into it.
Arm64 does not need the legacy MBR/BIOS install step (grub-install --target=i386-pc). Apart from that, the EFI install command line is the same as x86-64, except for the target arch (arm64-efi vs x86-64-efi).

TODO: Some Arm embedded platforms do not have runtime EFI variable support (usually those with U-Boot). grub-install will exit with an error, but the install does complete successfully otherwise. These boards will work as they search the removable media path.
I would like to figure out a way to determine when NVRAM support is present and "ignore" such errors.