Migrate to git-based kernel source trees

Using Portage to manage the Linux sources provides few benefits while becoming slower and increasingly incompatible with the way I want to do image-based updates. Fork the kernel sources like:

• torvalds -> nest/forks/linux/torvalds
• raspberrypi -> nest/forks/linux/raspberrypi

such that Puppet can use a vcsrepo resource to pull down a tag/version specified in Hiera to /usr/src/linux. Write the package version to package.provided to satisfy Portage.