tools: kwboot: Correctly set configuration of UART for BootROM messages

For kwbimage v1, tell BootROM to send BootROM messages to UART port number
0 (used also for UART booting) with default baudrate (which should be
115200) and do not touch UART MPP configuration.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>