Due to the recent change detailed in news item posted 2016-08-11 regarding the grub's naming, genkernel on the grub2 target no longer has the correct executable name for running grub-mkconfig. Since its apparently a multislot setting that the user can configure, I'd suggest genkernel be more flexible and detect it with a bit more detail.
*** This bug has been marked as a duplicate of bug 591200 ***