Update the kernel configuration section to cover 6.1:

• Update the lists of device drivers and tweak a few comments.
• Add a couple of missing options, like PREEMPTION and kbdmux.
• Remove some SCHED_ULE information. It's not stable and we shouldn't encourage users to make the switch, at least not right now.
• Remove some 386-specific info. We can't keep old stuff like this around forever, so let's nuke it.

This is not perfect yet though. There are still some
differences between GENERIC in 6.1 and this document.

Reviewed by: simon