Handle UEFI systems when rebuilding Grub 2 config file.

On UEFI systems, grub.cfg is in /boot/efi/EFI/<os>/grub.cfg instead
of in /boot/grub2/grub.cfg.  Luckily, instead of having to know what
"<os>" needs to be instantiated with (e.g. "centos"), it seems that
we can just point grub2-mkconfig to the symlink /etc/grub2-efi.cfg.
(This at least works on RedHat:ish systems.)