feat(profiles): Add coreos/amd64/usr profile.

This profile enables the symlink-usr USE flag and target profiles have a
new variable COREOS_DISK_LAYOUT_SUFFIX that allows the profile to switch
to a different set of disk layouts. By default no suffix is used but the
usr profile uses layouts with the suffix "-usr" such as "base-usr".
Merge pull request #293 from marineam/switch-usr

feat(profiles): Add coreos/amd64/usr profile.