binman: Take account of skip-at-start with image-header

The image-header currently sets it offset assuming that skip-at-start is
zero. This does not work on x86 where offsets end at 4GB. Add in this
value so that the offset is correct.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Bin Meng <bmeng.cn@gmail.com>
1 file changed