Automatically Locate Casper Directory

[mmstick]

It doesn't need to be hard coded.

[tintou]

A few things to change to make it more memory efficient and future-proof.
Don't forget to remove the option from the meson_options.txt file

[tintou]

looks good to me

[tintou]

@codygarver adding you as a reviewer as you better know if that's what you want

[davidmhewitt]

I'm happy that this will still work in the context of both the new and old ways of us building .iso images, so this should be fine to merge.