I have a workaround to fix the compiling of unionfs in the initramfs/initrd/kernel with genkernel. I don't know how to make an actual patch but /usr/share/genkernel/gen_compile.sh needs changes made, so I am just going to post my modified file which works fine for me.
Created attachment 84756 [details]
My modified /usr/share/genkernel/gen_compile.sh
*** This bug has been marked as a duplicate of 129910 ***