Instead of having a separate liftoff config, which is tested against
the default (which currently means tier-up from liftoff to turbofan),
just choose reasonable liftoff configs for the existing configs.
'ignition' now implies pure liftoff execution.
'ignition_turbo_opt' always compiles with turbofan.
Other configs use the default (tier up).

R=machenbach@chromium.org

Bug: chromium:824098, v8:6600
Change-Id: I92c008fc1b1fa54d3161fb5695a095127d6ac263
Reviewed-on: https://chromium-review.googlesource.com/1141731Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54548}