When LAUNCH_TERMINAL is not set, `make run` hangs with the following
messages:

 soc_term: save_current_termios: tcgetattr: Inappropriate ioctl for device
 soc_term: save_current_termios: tcgetattr: Inappropriate ioctl for device

The reason is an invalid test in common.mk.

Fixes: 07580605 ("common.mk: allow user to override launch-terminal in make run")
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>