Make inclusion in other projects easier

[akaihola]

I've been using Super UEFIinSecureBoot Disk which includes glim in its default full installation.

The version there is a bit outdated (see ValdikSS/Super-UEFIinSecureBoot-Disk#26), and it turned out to be a bit tricky to upgrade it (see ValdikSS/Super-UEFIinSecureBoot-Disk#27), especially due the assumption that inc-*.cfg files are in the same directory as the root GRUB configuration. This is not true in Super-UiSB-Disk since its root grub.cfg loads glim from a subdirectory using configfile "$prefix/glim/main.cfg" and puts inc-*.cfg in that subdirectory as well.

I'm not sure about on which side it makes more sense to tweak the approach, so I wanted to give a heads up of this problem in both and cross-reference the issues.

Thanks for the amazing work on glim @thias!