tools: kwboot: Make kwboot more robust on a38x

This patch accomplishes 2 things to make the kwboot procedure
on the a38x more reliable.

1)  We fill the tty with 1K of the magic bootparam.  This helps
with the timing of where the microcode picks up in the read of
the line to ensure we actually catch the break to go into recovery
mode

2)  Before starting the xmodem transfer we sleep for 2 seconds
and then flush the line.  This allows all the magic bootparam
to be flushed from the line and makes the xmodem transfer reliable
and removes the Bad message failures.

Signed-off-by: Jon Nettleton <jon@solid-run.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Stefan Roese <sr@denx.de>