[arch-releng] Add failsafe options

Quite often on some systems I get the problem that the screen stays black when booting an archiso because some driver snatches the display and then doesn't do anything. I think following other distros here would be a cool thing. A failsafe option for BIOS/EFI should be introduced that will for now only add "nomodeset" to the command line. This might help many users who would otherwise be frustrated. It's also not quite obvious to find a solution for this on Google. Thoughts?