[media] rc-core: add loopback driver

This patch adds a loopback driver to rc-core which I've found useful for
running scripted tests of different parts of rc-core without having to
fiddle with real hardware.

Basically it emulates hardware with a learning and a non-learning
receiver and two transmitters (which correspond to the two
receivers). TX data that is sent is fed back as input on the
corresponding receiver, which allows for debugging of IR decoders,
keymaps, etc.

Signed-off-by: David Härdeman <david@hardeman.nu>
Acked-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>