nftables: making interactive console optional

This lets you (by default enabled) compile out its readline
dependency.

[Thomas:
  - remove the patch, which is now unneeded, since we've bumped to
    nftables 0.4, which as the patch to make readline optional.
  - remove the new Config.in option, just enable the interactive
    console when the readline package is enabled.]

Signed-off-by: Alexander Clouter <alex+buildroot@digriz.org.uk>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>