Don't always enable MTD mode unless MTD device is specified

On MTD devices, don't always enable MTD mode - the installer needs to
be able to access both the GPT disk and the MTD disk, so only
enable it if we install to /dev/fts

BRANCH=none
BUG=chromium:221745
TEST=make runtests

Change-Id: I7688e6bc758ef47cfb9d468c1224ef43b2043d02
Reviewed-on: https://gerrit.chromium.org/gerrit/62662
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Commit-Queue: Albert Chaulk <achaulk@chromium.org>
Tested-by: Albert Chaulk <achaulk@chromium.org>
4 files changed