main/postmarketos-base: move os-release to /usr/lib
From https://www.man7.org/linux/man-pages/man5/os-release.5.html:
/usr/lib/os-release is the recommended place to store OS release
information as of vendor trees. /etc/os-release should be a
relative symlink to /usr/lib/os-release, to provide
compatibility with applications only looking at /etc/. A
relative symlink instead of an symlink is necessary to avoid
breaking the link in a chroot initrd environment such as dracut.