initial import of overlayfs squashfs support

Hello. I'm writing to mailing list about read/write support of rootfs.
Can you check my code? 
Firstly as i see dracut does not have generic support for fetching squahfs and various backends (dm, unionfs, aufs, overlayfs). Does it possible to create more generic livenet module, that based on some rd.live flag runs specific backend mount?

Signed-off-by: Vasiliy Tolstov <v.tolstov@selfip.ru>