sd-boot: Improve key bindings

sd-boot: Improve key bindings

Making keys case insensitive should help if caps lock is on.
We are not advertising them at runtime or in the manual to
reduce the noise.

This also hides the quit and version commands from the help
string. They are mostly for devs and otherwise have little
to no use to normal users. The latter overlaps with print
status which is still advertised.