tests/futility/test_update: Dynamically calculate image size

Stop hardcoding the image size.

BUG=none
TEST=make runfutiltestscripts
BRANCH=none

Change-Id: I796d81c22145faa01fcf9f34a055ad2a3b97267e
Signed-off-by: Yu-Ping Wu <[email protected]>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/7206724
Reviewed-by: Jakub "Kuba" Czapiga <[email protected]>
1 file changed