Add a "no fallback" option for unified kernel images

[MarkGhanz]

describe the request

My laptop is pre installed with windows with the ESP being only 260MB large, and it is very inconvenient to make it larger. As archinstall defaults with fallback image, the fallback UKI would make the ESP run out of space and cause the installation to fail. But without that fallback UKI the ESP is large enough. It would be very helpful to have an option to ignore fallback image at first so archinstall would be suitable for smaller ESPs.