fix(build_library): make the usr link relative

Fix the problem of: "pecified switch root path %s does not seem to be an
OS tree. /etc/os-release is missing" because dracut doesn't have an
/usr/share/coreos/os-release file.
fix(build_library): use ln -sf instead of if

we don't need an if statement when we can force.
fix(build_library): put lsb-release in usr too
fix(build_library): mkdir /usr/share/coreos in the right order

This is important otherwise things fail to create files because the
directoreis don't exist.
Merge pull request #191 from philips/relative-link

Fixup all of the symlinks in set_lsb_release