fix(sys-libs/nss-usrfiles): Drop unused data files.

Should have been dropped in 46c02b02 but was missed.
feat(sys-apps/baselayout): New CoreOS specific baselayout package!

This replaces the old Gentoo baselayout and coreos-base packages.

Changes include:
 - Move nss data files from /etc to /usr/share/baselayout
 - Enable nss-usrfiles module to use the new location.
 - Move other misc files from /etc to /usr/share/baselayout, using
   compatibility symlinks in /etc generated by tmpfiles.
 - All base system directories can be generated by tmpfiles.
 - No more /etc/gentoo-release
 - Simplified code, doesn't bother trying to migrate lib symlinks and
   simply fails if the existing filesystem is incorrect.
 - In /usr images the `core` user's UID/GID is now 500 to keep us within
   the reserved system UID/GID space. Eventually once the SDK switches
   to this the `core` user will not conflict with the local developer's
   account. It also makes it clearer what range people can use when
   creating accounts in /usr images. No other UID/GIDs are changing.
 - New eclass to let ebuilds run the equivalent of `tmpfiles --create`.
   In the future this may be replaced by calling `tmpfiles` directly
   once it has a `--root` argument but I haven't pushed those patches
   upstream for review yet.
maint(eclass/*): Delete a whole pile of old unused eclasses.
fix(sys-apps/baselayout): Assorted cleanups/fixes

- Add || die to commands without them to avoid missing errors.
- Symlink resolv.conf to /run on amd64-generic images again.
- Properly sed /etc/issue out of tmpfiles.
- Fix symlinks for mtab and sudo.
- Fix directory ordering in tmpfiles.
- Update groups, a few were missing or incorrect.
- Bump coreos-base/coreos revision.
Merge pull request #359 from marineam/baselayout

Baselayout