build: Add LINUX_KERNEL_COPYING license file

The build requires LINUX_KERNEL_COPYING when building
the notices. Change from depending on a file in
prebuilds/qemu-kernel to one in /build/core.

This is required as Brillo does not have the qemu-kernel
in its checkout.

BUG=25578534

Change-Id: Ieff6970035d27884c0769106e507284c1096c33f