use explicit target arch for GRUB2 and remove grub2-efi calls

bnc#782891 unified grub2 and grub2-efi. Remove all references to
grub2-efi-* and use explicit target argument where appropriate. After
this change GRUB2.pm is for legacy BIOS boot and GRUB2EFI.pm is for
UEFI boot. It is also possible to switch between them by explicitly
selecting bootloader type (grub2 vs. grub2-efi).

Signed-off-by: Andrey Borzenkov <arvidjaar@gmail.com>