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
diff --git a/core/Makefile b/core/Makefile
index 6c07067..237d5d1 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -664,7 +664,7 @@
 # make the target NOTICE files depend on this particular file too, which will
 # then be in the right directory for the find in combine-notice-files to work.
 $(kernel_notice_file): \
-	    prebuilts/qemu-kernel/arm/LINUX_KERNEL_COPYING \
+	    $(BUILD_SYSTEM)/LINUX_KERNEL_COPYING \
 	    | $(ACP)
 	@echo Copying: $@
 	$(hide) mkdir -p $(dir $@)