libweave: import files kept in platform2

Since libweave isn't used outside of CrOS anymore, merge the platform2
fragments back in here so we can clean up platform2 entirely.

BUG=None
TEST=build still works

Change-Id: I80163c45c582f08465d36b7f26a7809aed60cd03
Reviewed-on: https://chromium-review.googlesource.com/1187078
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>