tools: kwboot: Check if baudrate value is supported before sending image

Call kwboot_open_tty() which baudrate value which was specified at the
command line by option -B. This function returns error if baudrate is not
supported by selected tty device.

Initial baudrate for image transfer is always 115200, so call
kwboot_tty_change_baudrate() with value 115200 immediately after
kwboot_open_tty() if baudrate specified by option -B is different than
115200.

This makes kwboot fail immediately, informing that baudrate is unsupported,
instead of failing only after the first part of image is already sent.

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