kernel: Fix the check for official builds

The check for official builds now only affects the case where a prebuilt
kernel was actually used, not the case where a device builds a kernel
from source as normal.

The check has also been turned around so that it only fails the build if
it's actually an official build, not an unofficial one.

Change-Id: I1280b16b90589742c2abe7f3f6f5975a3aaeea03