Update trunks, attestation, feedback to depend on libchromeos

Libchromeos is now a standalone package. packages should depend on it with the
deps variable mechanism instead of depending on the gyp target.

BUG=chromium:381372
TEST=USE="attestation" emerge-amd64-generic platform2 works.

Change-Id: Idbe2e730c9f33e32f5154553d7e3f32ad4c04de9
Reviewed-on: https://chromium-review.googlesource.com/204688
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>