kernel: assume modules.builtin is always present

We do not support old kernel versions not generating modules.builtin
anymore, so assume it will always be present and check for modules
to be built in first.

This prevents old modules being packages up after changing the kernel
config to include them in the kernel without cleaning the kernel tree.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46182 3c298f89-4303-0410-b956-a3cf2f4a3e73