https://git.yoctoproject.org/cgit/cgit.cgi/poky/tree/meta/recipes-kernel/linux/linux-yocto-tiny_4.18.bb#n2 has a typo. It should refer to the 4.18 branch, not 4.15
A patch was already sent for this, but it looks like it fell through the cracks. I'll send it again with my next series.
This was fixed on master.