add(10diskless-generator): Add generator for squashfs and tmpfs mounts.

This replaces our short-lived use of fstab in the initrd to setup
diskless systems because overriding fstab with kernel command line
options doesn't work properly. A PXE system should be capable of using
the /usr squashfs image but a root file system on local disk.
fix(99dracut-root): Replace 99fstab with an even more stupid hack.

Dracut's explicit check to see if one of its own modules has sorted out
the root filesystem is pointless since if nothing mounts the root
filesystem the initrd is going to fail and drop into emergency mode
anyway. This pacifies dracut and lets our systemd generators work.
add(remount-sysroot.service): If needed remount root as rw

If the /usr directory does not exist in the root filesystem it will need
to be created before /usr can be mounted. Once -generic is gone we can
just change the default to always mount root as rw instead of ro.
Merge pull request #31 from marineam/diskless

Fix using root= on diskless systems