kernel: Fix packaging issue

Remove /etc since it is empty, when creating a machine that does not
deliver any module config files, the /etc is empty and is then warned
about not being shipped, so we remove it.

This occurs in the routerstationpro with the following warning:
WARNING: For recipe linux-yocto, the following files/directories were installed but not shipped in any package:
WARNING:   /etc

Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>