netboot: Introduce support for nbd protocol v3

The plan is to add a new cmdline argument ''nbd.v3' that could be parsed by the initrd script used for netboot.

• if nbd.v3=1 , then script will try to use newstyle nbd protocol. It will request an image named "default-image", or another one if specified by argument "nbd.image=....."
• otherwise, we fall back to the legacy client (implemented with busybox, no image name required) - this is the current required mode for CI.

Amongst other features, the new nbd protocol will allow to serve different images to multiple boards with only one nbd server.