kbuild: Only build linux-image package for UML

For user-mode Linux the other packages are not required. So
only build the package with the linux-image in it.

Cc: maximilian attems <max@stro.at>
Cc: debian-kernel@lists.debian.org
Cc: linux-kbuild@vger.kernel.org
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Tested-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Michal Marek <mmarek@suse.cz>