dracut rootfs overlayfs module

Make any rootfs ro, but writable via overlayfs.
This is convenient, if for example using an ro-nfs-mount.

Add the parameter "rootovl" to the kernel, to activate this feature

This happens pre-pivot. Therefore the final root file system is already
mounted. It will be set ro, and turned into an overlayfs mount with an
underlying tmpfs.

The original root and the tmpfs will be mounted at /live/image and
/live/cow in the final rootfs.