scripts: kwboot: set boot source to UART before sending
Sending an image that specifies one of the alternative boot sources doesn't
make sense. So change the boot source to UART on the fly.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>