kernel-module-split: Allow for external conf files

kernel-module-split: Allow for external conf files

Some recipes might provide conf files produced during build phase or
simply tracked in the VCS instead of generating them with Yocto.
In such cases those conf files wouldn't be assigned to correct packages.
With this change, if user wants to generate a conf file they still can,
but not generating them won't prevent assigning the file to proper
package given the file exists.

Signed-off-by: Michal Sieron <michalwsieron@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>