greybus: loopback_test: handle SIGINT signal

Adding a default timeout may not be representative of every
usecase for gb_loopback. Also, tests may continue to run
on the driver in case of a timeout.

To avoid adding a default timeout, handle SIGINT so that when the user
presses ctrl-c the test are stoped. The user can still specify a timeout
value with the -O option.

Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>