include: group kmod ipk files into a "kernel" subdirectory

This is useful to just use the kmods from an official build while supplying
base packages from a custom feed or the other way around; for just overriding
the kmods with a local repo while using official repos for the rest.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 48475