Positional size argument should become a flag

[sowbug]

It would be more idiomatic for the firmware-config-size positional arguments to be like this:

mkosimg [-s SIZE] firmwareimage [configtarball]

Then we won't have to do the less idiomatic - to skip the config file argument. This isn't a big deal because I'm sure most users will include the config file -- after all, creating a preconfigured image is the main value of this project to begin with.