kbuild: Change section of generated debian packages to kernel

To follow the way that Official Debian kernel packages are made, put the
generated packages in the right section, the kernel section.  This also
avoids polluting the admin section.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
Signed-off-by: Michal Marek <mmarek@suse.cz>