boards: arm: mps2_an521: Enable qemu host tty

This configure and connect host /dev/pts/N to arm qemu uart-1. It can
use a real device like /dev/ttyS0. Just switch config between serial
and pty chardev options.

In addition, this update QEMU_EXTRA_FLAGS to use only variables instead
hard coded values.

Signed-off-by: Gerson Fernando Budke <gerson.budke@ossystems.com.br>