flash-img-sample.sh: Allow flash-imgs-sample.sh to take device argument
flash-img-sample.sh: Allow flash-imgs-sample.sh to take device argument

Use a sane default, but allow a tty device to be specified
by the user when calling flash-imgs-sample.sh

Signed-off-by: John Stultz <33837adfb53316eae44aad04041e1240088d76fd@linaro.org>

96boards-hikey/tools-images-hikey960,96boards-hikey/tools-images-hikey960