feat(set_lsb_release): Add special "sdk" track without an update server.

I want to start including version info in SDK builds as an alternative
scheme to the existing "chroot_version_hooks" system which always
assumes freshly unpacked SDKs are the latest regardless of what version
they actually were.
fix(set_lsb_release): Make sure $ROOT/etc exists.

Useful for generating a root filesystem overlay that catalyst can apply.
fix(bootstrap_sdk): skip final release goo if stage4 wasn't built
feat(bootstrap_sdk): Add release data to SDK tarballs.
Merge pull request #79 from marineam/lsb4sdk

Add lsb-release to SDK tarballs