Put build configuration in seperate gitignored file

[holzingk]

The path to the toolchain depends on the host. Therefore it should not be part of version controlled files.

The toolchain could either be specified in a seperate file in the repository or globally somewhere in the ~/.config/grisp/ directory.

[sylane]

We could just add support for system variable substitution in the path...

[eproxus]

@sylane Then the users would just have to maintain environment variables somewhere else. I'd prefer a separate file.