nbd: new package nbd-server

nbd: new package nbd-server

Network block device server allows to export a block device from router
to remote host. This is particularly useful if no network filesystem
server is feasible or direct access to a block device is needed.

It's been tested for nearly a month on ar71xx (TL-WR842ND) and proved to
be very stable and efficient solution.
The package comes with init.d script and conf.d file allowing to
configure most nbd-server options using standard uci interface.

Signed-off-by: Marcin Jurkowski <marcin1j@gmail.com>