cleanup(80setup-root): Remove ${BOOTENGINE_ROOT_DIR} var

This module doesn't have a test harness which overrides the var so
unclutter the code, just refer to /sysroot instead.
fix(80setup-root): Silence "machine-id: No such file" error message
fix(80setup-root): Handle initrd OEMs

Replaces the copy currently handled by the squashfs module.
add(99fstab): New module to satisfy dracut without root=

The PXE initrds can simply provide a fstab to mount the squashfs instead
of requiring users to specify kernel parameters. Dracut needs a little
nudge to feel OK about it though.
remove(80squashfs): Replaced by using fstab instead.
Merge pull request #28 from marineam/usr

Booting PXE images