install kustomize via hack dir

[gianarb]

No description provided.

[deitch]

Minor change to make it more user-friendly.

[deitch]

Can we have a "human-readable" dependency, so it looks something like:

kustomize: $(KUSTOMIZE)
$(KUSTOMIZE): $(TOOLS_DIR)/go.mod # Build kustomize from tools folder.
 	cd $(TOOLS_DIR); go build -tags=tools -o $(BIN_DIR)/kustomize sigs.k8s.io/kustomize/kustomize/v3

and then for the later dependency:

manifest: kustomize semver release-manifests release-clusterctl release-cluster-template

and then we can have a list of tools:

tools: kustomize controller-gen kubebuilder

This makes it easier to read, easier to parse, and one can do:

make kustomize

to ensure it is installed.