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.