tools: kwboot: Allow to specify custom baudrate only in supported operations

Custom baudrate different than 115200 may be specified only when kwboot is
not going to send boot/debug message pattern or when it is going to send
boot message pattern with image file (in which case baudrate change happens
after sending kwbimage header). BootROM detects boot/debug message pattern
only at baudrate 115200.

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