diskimage-builder, or at least the most common elements, have
always had an implicit requirement to be run code intended for a
linux machine on the targeted architecture. This documents that
requirement up-front so users won't attempt in vain to build
cross-architecture or use it unix-like systems like Mac OS.
Change-Id: I22be1f9c929054c8ddc3253f55fa310bee29c055
ce724871ae