futility: update: Preserve RW_ELOG
futility: update: Preserve RW_ELOG

The eventlog is stored in RW_ELOG and should not be destroyed during firmware
update.

BUG=chromium:655423
TEST=make futil; tests/futility/run_test_scripts.sh $(pwd)/build/futility
BRANCH=None

Change-Id: I0b4fe1e78ae2d499e985471f51cfa9eb983c8adc
Signed-off-by: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1242663
Reviewed-by: Julius Werner <4d12f06cff3725783b352157673a720bc67c4afe@chromium.org>

coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot