feat: add build and rootdrive our init

[Joe-Degs]

There is this additional idea I am getting 😆 , you are working on the init and using spitfire to create the root drive after every change for rapid testing. you probably want to just build the thing from your local working repo and then test it out to see if it works. so now I introduce the --build-from command. it let's you specify the local repo that contains the init code and you just build and run.

We end up getting something like spitfire mkrooot --size 100M --fs ext4 --init --build-from ~/thi-startup/init

[Joe-Degs]

so new commit changes a couple of things.

• we have better documentation on the commandline now
• --init's default behaviour is installing from github but with --build-from you can specify a local repo to use instead
• i think I have cleaned up the code a bit but its still shit so you might not notice 😆