update-libre.sh doesn't commit by default so that it can be used as an
updateScript, where I don't think auto-committing is the norm.
The generated commit messages say "linux-libre_latest" rather than
"linux-libre", because even though linux-libre will also be rebuilt,
it's linux-libre_latest that is more likely to need it.
- defined buildLinux as generic.nix instead of manual-config.nix. This
makes kernel derivations a tad more similar to your typical derivations.
- moved $buildRoot to within the source folder, this way it doesn't have to be created before the unpackPhase
and make it easier to work on kernel source without running the unpackPhase
