tools: kwboot: Fix sending very small images

tools: kwboot: Fix sending very small images

Sending of very small images (smaller than 128 bytes = xmodem block size)
cause out-of-bound memory read access. Fix this issue by ensuring that
hdrsz when sending image is not larger than total size of the image.
Issue was introduced in commit 20a6585838ca ("tools: kwboot: Fix sending
Kirkwood v0 images"). Special case when total image is smaller than header
size aligned to multiply of xmodem size is already handled since that
commit.

Fixes: 20a6585838ca ("tools: kwboot: Fix sending Kirkwood v0 images")
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>