whilst some parts of the codebase were set up to work with multiple types
specified, others did not work with it and would not necessarily be easy
to adjust. this thus makes some tweaks to adjust things accordingly.
- option renamed to singular form (maintaining backwards compatibility)
- a validation check has been added
- unnecessary glob style type references fixed
- checks with In_list changed to a direct singular comparison
- typo of type "netboot" written as just "net" fixed (though unreachable
so of no consequence; really the code could be removed but it's trivial)
Gbp-Dch: Short
39e4d3e3cb