Update KERNEL_VERSION to match buildroot config

We downgraded the kernel (to 4.15) in 2be51dc,
but never updated the Makefile from 4c4cd31...

The default kernel in Buildroot 2018.05.x was
4.16.13, but upgraded in Minikube to 4.16.14

Makefile

@@ -43,7 +43,7 @@ MINIKUBE_BUCKET ?= minikube/releases
 MINIKUBE_UPLOAD_LOCATION := gs://${MINIKUBE_BUCKET}
 MINIKUBE_RELEASES_URL=https://github.com/kubernetes/minikube/releases/download
 
-KERNEL_VERSION ?= 4.16.14
+KERNEL_VERSION ?= 4.15
 
 GO_VERSION ?= $(shell go version | cut -d' ' -f3 | sed -e 's/go//')
 GOLINT_VERSION ?= v1.17.1