fix(remotes): Rename coreos remote to just github.

All the paths prefix with coreos so the remote isn't limited to
repositories under the coreos user. But more importantly I want to add
the coreos/linux repository to the manifest so it gets checked out
during repo sync instead of during build_packages and it uses the prefix
'coreos/' on all branch names and that makes coreos/master ambiguous
since it could mean refs/heads/coreos/master or
refs/remotes/coreos/master or refs/remotes/coreos/coreos/master...

Change-Id: I4850dcdf9954e34d90e7cc4402d1960ee09428a6
feat(full): Add linux repo to manifest.

Adding to manifest so it is fetched up-front and shared between every
SDK chroot someone might create instead of cloning during
build_packages. Considering it can take longer to clone than actually
compile this should be an improvement. Use the minilayout group if you
prefer the old behavior or don't need to do package builds.

Change-Id: Ia665a26d2e64892e8c0fc7f1e91eac057987bcba
Merge pull request #11 from marineam/kernel

Add kernel to the manifest