Without this change, trying to create a system image with efi-raw type while
using grub-efi-removable-bootloader fails with fairly confusing message:
EFI bootloader required with GPT partitioning
* gnu/system/image.scm (system-disk-image): Consider
grub-efi-removable-bootloader to be EFI bootloader.
Change-Id: I5f5e1a94e825fd29d6880c5bafb330e16a5ac962