buildroot: Add netcat to the busybox build

This gives a light weight way to copy files between the host and
target. This can be used with user networking on the fvp.

An example of its use:
On target # nc -l -p 5555 > hello.txt < /dev/null
On host   $ cat hello.txt | netcat localhost 5555

Signed-off-by: Ben Horgan <ben.horgan@arm.com>
Change-Id: Ic2f44300ba6c12eb9f6f94c5e2c13e675cfb2c48