Allow running MOK generation as a command

[scaronni]

Allow calling part of prepare_signing as a standalone function to generate the MOK key as a standalone command.

This is for the use case described here: https://gitlab.gnome.org/GNOME/gnome-software/-/merge_requests/2034#note_2147952

$ sudo dkms generate_mok
Signing key: /var/lib/dkms/mok.key
Public certificate (MOK): /var/lib/dkms/mok.pub

This is the minimal change to get the command available. Open to suggestions if we should instead rework the whole signing logic.

[scaronni]

@evelikov or @xuzhen any chance for a review and merge?

[zeroepoch]

I'm a relatively new reviewer to the DKMS project so I'm not aware of the full scope of changes required to add this new feature (if something was overlooked and missing), but the changes proposed in this MR look reasonable and not introducing any obvious bugs.