Following up on https://bugs.funtoo.org/browse/FL-5999?focusedCommentId=39737&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-39737, which clarified that only debian-sources(-lts) are supported kernel sources in Funtoo 1.3, all other kernel sources should be masked (same as other unsupported catpkgs like gcc 8, etc.) so as to avoid users unknowingly install an unsupported kernel.