handle late filesystems

[q66]

Currently no effort is made to mount network/auxiliary filesystems. We should probably have another stage that will run. Some thought will need to go into ordering/making it generic, because in some cases there may be cycles.

[dropwhile]

@q66 Were you thinking of something like a network-fs target that would use a helper script similar to fs-fstab.sh but would use -O _netdev instead?