Kernel: never provide a development prebuild package

Even if the developer enables the usage of prebuild packages, the Linux kernel
should not be part of it. The kernel is a too sensitive part and its build must
always be forced.

Signed-off-by: Juergen Beisert <jbe@pengutronix.de>
Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>