Allow local non-git sources

See merge request redox-os/cookbook!250

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.

Fix broken xz recipe

See merge request redox-os/cookbook!247

Caused by redox-os/cookbook!246

Remove customization of and update xz.

See merge request redox-os/cookbook!246

I updated the recipe to use newest the raw xz upstream source and
cleaned out my recipes and rebuilt. I have detected no problems.
Not having to have customizations will make it easier to upgrade
int the future as new versions are released.

Signed-off-by: Wren Turkal <wt@penguintechs.org>

Compile gnu-binutils with pie so it works on Redox

See merge request redox-os/cookbook!245