tools: kwboot: Add xmodem timeout option

Add command-line specification of xmodem timeout.  If the binary
header needs to take a while to do something (e.g. DDR ECC
scrubbing), the xmodem transfer can time out.  Add a configurable
xmodem block timeout to allow transfers with slow binary headers
to succeed.

Signed-off-by: Kevin Smith <kevin.smith@elecsyscorp.com>
Cc: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>