tools: kwbimage: Add support for new commands UART_PORT and UART_MPP

These two commands allow to specify custom setting of UART port used for
printing BootROM messages.

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