cmake: emu: Allow QEMU to serve serial on a TCP/IP port

This commit enables users to pass -DQEMU_SOCKET=y for 'west' when
selecting to run the build on a QEMU VM. This allows the target's serial
port to be served to the host via a TCP/IP port for convenience, easing
communication with the target from the external world.

Signed-off-by: Erik Schilling <erik.schilling@linaro.org>
Signed-off-by: Gustavo Romero <gustavo.romero@linaro.org>