Update squashfs hook - add NFS support

* add new kernel option "squashfs_source_opts"
* add PARTLABEL, PARTUUID support for local image source
* both remote and local sources will be mounted in read-only mode by default
* move image search part to function squashfs_source_postmount
* add NFS support (all versions)
* change default image path to /squashfs/root.squashfs