buffet: Add privet code into buffet binary

Code is still not called, only compiled and unit tested.

BUG=brillo:1161
TEST=`FEATURES=test emerge-gizmo buffet`

Change-Id: I2ba06200698bbec324d3e1598db1c8e261062a39
Reviewed-on: https://chromium-review.googlesource.com/276365
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Tested-by: Vitaly Buka <vitalybuka@chromium.org>