Adding 'N:' field support to buildman's boards database

Hey,

So looking at series such as https://patchwork.ozlabs.org/project/uboot/patch/20220801113804.v2.1.I49e41e... we run in to the case that instead of using "F: configs/..." to list each defconfig, we use the "N:" syntax, which is to list a regex of files for the given MAINTAINERS entry. We need to support this format in order to continue to have CI pass once we merge the above mentioned series.