This makes it easier to make driver changes. And in the future other
recipes could potentially benefit from the same infrastructure too.

This make a simple make r.initfs enough to rebuild the initfs when
changing drivers. Previously make cr.initfs was necessary which rebuilds
redox-initfs-ar from scratch.

Sha256 is slower than blake3 and all recipes use blake3 now.

This is both faster and more correct.

It is unused.

This makes the source section of recipe.toml optional, so that one can
simply create a recipe with a build recipe and a source folder and not
have to worry about git at all just to get a program running in redox.

I also ran into a situation where a source directory without a git
repository in it caused cookbook to reset it's own origin URL to the URL
specified in the recipe for that source directory. I added a check to
prevent that happening to anybody else.

Needed to stop mass-errors for us mortals without the latest rust
version, ever since redox_syscall 0.1.57 was merged.