futility: cmd_update: Preserve image sections before update

When updating RO (or going to compare with contents), we need to
preserve (copy) section data from system active firmware image.

The `preserve_images` will try to preserve a list of known sections in
full update (`--wp=0`) mode, so we VPD data and HWID won't be lost.

BUG=chromium:875551
TEST=make futil; tests/futility/run_test_scripts.sh
BRANCH=None

Change-Id: I85c4ba972853dbc0fc101bee269c0effe70988b1
Signed-off-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1183653
Reviewed-by: Randall Spangler <rspangler@chromium.org>