tools: kwboot: Resend first 3 xmodem retry packets immediately

Currently when kwboot receive some garbage reply which does not understand,
it waits 1s before it tries to resend packet again.

The most common error on UART is that receiver sees some bit flipped which
results in invalid reply.

This behavior slows down xmodem transfer over UART as basically on every
error kwboot is waiting one second.

To fix this, try to resend xmodem packet for first 3 attempts immediately
without any delay. If broken reply is received also after the 3 attempts,
continue retrying with 1s delay like it was before.

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