lakitu: make update_engine optional feature

A lot of platforms won't use update_engine, e.g
kumo or anthos. Either they have to stub it or
use USE flag. USE flag looks the right way.

Did some cleanup by removing some TODO.

Nothing changes at functional level

BUG=b/171355221
TEST=sponge2/82e3ee60-03c7-4eb4-8365-974b5acaafd6
RELEASE_NOTE=None

Change-Id: I9cd9aaa75c29e7e5a6e9bd2820a2370d53d6551a
6 files changed