manifest: Stop tracking coreboot

* No one builds or uses this, was not tracked in
  14 or 15-rel, and when devices set flags needed
  for vboot, this is included.
* None of this would be a problem if the coreboot
  package actually built but it does not.
* Stop tracking it.

Change-Id: I1db5c4ca272e79eac94968f953a8a04875f14191