tools: kwboot: Check for return value of kwboot_tty_send() and tcflush()

Failure of kwboot_tty_send() and tcflush() functions is fatal, it does not
make sense to continue. So return error back to the caller like in other
places where are called these functions.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-by: Stefan Roese <sr@denx.de>