gnu: grub-efi-bootloader: Specialize grub-install invocation.

gnu: grub-efi-bootloader: Specialize grub-install invocation.

* gnu/bootloader/grub.scm (install-grub-efi): Fix grub-install invocation for
  EFI systems.
* gnu/system/examples/bare-bones.tmpl: Use the newer
"bootloader-configuration" syntax.
* gnu/system/examples/desktop.tmpl: Use bootloader-configuration sytax.  Also,
use the same label for the LUKS-mapped device and the root partition.  Remove
unneeded "title" field for the file-system based on LUKS; as noted in the
manual, the "title" field is ignored for mapped devices.
* gnu/system/examples/lightweight-desktop.tmpl: Use bootloader-configuration,
and use grub-efi-bootloader.