tools: kwboot: Update manpage

Document -D, -b, -d, -q and -s options.

Add common examples how to use kwboot.

Add information about Armada 38x BootROM bug for debug console mode and how
to workaround it.

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