Make elf path optional when listing devices

[therealfrauholle]

Currently the user must supply an elf path when listing devices. This argument is actually not used, the tool will exit after listing devices and can actually be a path not pointing to a file.

See therealfrauholle@f95ac87 for a reference, e.g. a hack that could solve this. This could already solve the problem, but we want to use clap to its full potential to nicely reflect the usage of elf path in the CLI.