tools: kwboot: Support higher baudrates when booting via UART

tools: kwboot: Support higher baudrates when booting via UART

Add support for uploading the boot image (the data part only) at higher
baudrate than the standard one.

The kwboot utility already has -B option, but choosing other baudrate
than the standard one (115200 Bd) can only work for debug mode, not for
booting the device. The BootROM for kwboot supported platforms (Orion,
Kirkwood, Dove, Discovery, AXP, A37x, A38x, A39x) cannot change the
baudrate when uploading boot image via the Xmodem protocol, nor can it
be configured via strapping pins.

So instead we add this support by injecting baudrate changing code into
the kwbimage v1 header as a new optional binary extension. This code is
executed by BootROM after it receives the whole header. The code sends
the magic string "$baudratechange\0" just before changing the baudrate
to let kwboot know that it should also change it. This is because the
injected code is run as the last binary extension, and we do not want
to loose possible output from other possible binary extensions that
came before it (in most cases this is U-Boot SPL).

We also inject the code before the payload (the data part of the image),
to change the baudrate back to the standard value, in case the payload
does not reset UART.

This change improves boot time via UART significantly (depending on the
chosen baudrate), which is very useful when debugging.

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