Revert "KCM Keyboard: resetOlbXkbOptions should not be a kconfig entry"

The option is required to be able to distinguish between "don't change options" and "reset options".

Bugfix but not cherry-pick because it's a behaviour change (back to good/expected behaviour, but a change nonetheless).

Two unrelated bonus fixes included, see commits for details. Those are suitable for cherry-picking.

Draft because it depends on !757 (merged).

CC @butirsky