Some EFI fixes

This merge request intend to fix several issues when a legacy grub is used in EFI mod. (eg: RHEL6)
- Grub shouldn't be installed on the MBR
- In efibootmgr, only one backslash is needed in the bootloader path to separate dirs
- When we add a rear entry to the GRUB config, adapt the path on EFI
- Fix bootloader name inside efiboot.img. It needs to be BOOTX64.efi, not a distro specific name.
- Don't use a GRUB2 config on a GRUB legacy bootloader in the bootable ISO