fix error for default hdd|netboot images

commit f8116561 enabled the grub-efi bootloader by default for amd64|i386 architectures, but failed to recognise the this bootloader is not supported for hdd|netboot images.

this meants that if a user tried to build such an image without explicitly specifying the bootloader, excluding grub-efi, their build would fail with an error in the binary_grub-efi stage.

this fixes the problem by only enabling grub-efi by default on supported image builds.