bootimg/grub-efi.bbclass: allow using a different class for EFI images

bootimg/grub-efi.bbclass: allow using a different class for EFI images

Abstract away some names so one can select using EFI_PROVIDER a different
class than grub-efi for populating live images, basically allowing the use
of a different bootloader than grub-efi.

Signed-off-by: Stefan Stanacar <stefanx.stanacar@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>