Merge pull request #397 from marineam/btrfs

feat(coreos-base/coreos): Add btrfs-progs and brctl
feat(app-emulation/docker): Add .network configs for networkd

These will match docker0 and the assorted veth* interfaces before the
default dhcp enabled network config does so networkd leaves them alone.
fix(sys-apps/baselayout): Create resolv.conf symlink except in SDK

networkd uses its own location for generating resolv.conf and a new
tmpfiles rule creates a symlink to it in both -generic and -usr images.
This is not applicable to SDK so remove the rule in that case.