Reland "iteflash: speed up data write"
Reland "iteflash: speed up data write"

This is a reland of 3430629ed4e993594d83e9ea22f984f2cf5fce76

Original change's description:
> iteflash: speed up data write
>
> In Cherry/Asurada, around 60% of the flash is not used. Skip writing
> empty contents makes flash_ec runs 2x faster.
>
> BUG=none
> TEST=Run `time util/flash_ec --board=cherry --image build/cherry/ec.bin`
>        Before this patch: 5min 54s
>        After:             2min 33s
>      And make sure the content is good.
> BRANCH=none
>
> Signed-off-by: Ting Shen <phoenixshen@google.com>
> Change-Id: I6aa5c3c00e2494b87a139f7a09aad9734e68c98d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2785510
> Reviewed-by: Dino Li <dino.li@ite.corp-partner.google.com>
> Reviewed-by: Eric Yilun Lin <yllin@chromium.org>
> Commit-Queue: Ting Shen <c671d3efec3d7e7e285f3788bf0d1ba4c8605b78@chromium.org>
> Tested-by: Ting Shen <c671d3efec3d7e7e285f3788bf0d1ba4c8605b78@chromium.org>

Bug: none
Change-Id: Icce2ffc404d0e83fa710a5fd1c201e7fbbf10101
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2796376
Tested-by: Ting Shen <c671d3efec3d7e7e285f3788bf0d1ba4c8605b78@chromium.org>
Reviewed-by: Aseda Aboagye <12c9b286316a940fd31c24070da5ab64cdd4d0e7@chromium.org>
Commit-Queue: Ting Shen <c671d3efec3d7e7e285f3788bf0d1ba4c8605b78@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec