[arch-releng] Add failsafe options

[Sven-Hendrik Haase]

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?