Created attachment 465450 [details, diff] Patch that adds a patch to genkernel for unionfs-fuse 0.24 unionfs-fuse 0.24 is an old version from 2010, maybe it would be better to use 0.26, however failing that, here is a one liner that should allow --unionfs option to compile in genkernel.
Thank you for the patch. Sorry that we didn't look into it in time. Since genkernel-4 it should be obsolete.