oeqa/utils/nfs: Add unfs_server function to setup a userspace NFS server

oeqa/utils/nfs: Add unfs_server function to setup a userspace NFS server

Add a nfs module into oeqa utils. This module provides unfs_server which
allows a test case to build unfs3-native and setup the unfs server on a
target directory of the host. This directory is then shared and can be
mounted by the host or a target device attached to the host (e.g. qemu
via tap or slirp). The nfs server is setup over UDP and automatically
assigns user privileged ports. The function provides the UDP ports for
the server as part of a returned python contextmanager which handles
cleanup of the server process on completion or exception.

Also add a 'udp' arg to get_free_port to get a free UDP port.

Note: unfs3 still requires the host to have rpcbind or portmap running.

(From OE-Core rev: c754fd85be85ad0a381b642365eca17cea8eb627)

Signed-off-by: Nathan Rossi <nathan@nathanrossi.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>