Fix 'local' to not look in the source tree for the file.

[bdrewery]

Summary

Usually 'local' is used along with other rules such as 'no-implicit-rule' or
'dependency' which avoids this problem. It's possible to need to use 'local'
while relying on the default rules though for a file which is not in the source
tree nor generated in the kernel.

Sponsored by: Dell

Test Plan

This has no current impact on FreeBSD KERNCONFS. I did a buildkernel
of amd64 GENERIC and have no problems.

Tested this downstream where we have our own files listed in sys/conf/files
that are generated outside of buildkernel. With "local" used now they
are properly being found.

[bdrewery]

This has a minor conflict with your D13124 change.

[imp]

this sounds reasonable to me...