Added file: /live-files/general-files/etc/default/grub

This would alter behavior of grub after a live install, making the previously booted entry automatically selected at each reboot. Also flat menu entries are enabled, which is desirable in a 'init-diversity' multi-init set-up.

I have created a seperate 'init-diversity' branch for this in my fork, as at this point I'm unsure if @anticapitalista would endorse this for a single init system.