tools: kwboot: Allow greater timeout when executing header code

tools: kwboot: Allow greater timeout when executing header code

When executing header code (which contains U-Boot SPL in most cases),
wait 10s after every non-xmodem character received (i.e. printed by
U-Boot SPL) before timing out.

Sometimes DDR training, which runs in SPL, may be slow.

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