grub.cfg: move ubuntu (secboot) grub cfg to /boot/grub

Usually ubuntu ships a grub.cfg in /boot/grub/x86_64-efi itself that
loads some modules (depending on the ubuntu image!) and then loads
the grub.cfg in /boot/grub. So, replace the config in /boot/grub instead
of /boot/grub/x86_64-efi. lipck will copy the original config in
/boot/grub/x86_64-efi and thus, the ubuntu grub will load the
correct modules and then load our config.