chore: Small Makefile fixes (red-hat-data-services#928)

* Go back to using `IMG ?=` rather than forcing the value with `IMG =`. This makes it easier to override the value of `IMG` for development, and it also aligns it to how `BUNDLE_IMG` is assigned.
* Add `prepare` as dependency of the `undeploy` target. This is just for convenience, although when looking at the other targets, `prepare` may simply be a missing dependency in `undeploy`.

Signed-off-by: Edgar Hernández <23639005+israel-hdez@users.noreply.github.com>

Makefile

@@ -14,7 +14,7 @@ IMAGE_TAG_BASE ?= quay.io/opendatahub/opendatahub-operator
 # keep the name based on IMG which already used from command line
 IMG_TAG ?= latest
 # Update IMG to a variable, to keep it consistent across versions for OpenShift CI
-IMG = $(IMAGE_TAG_BASE):$(IMG_TAG)
+IMG ?= $(IMAGE_TAG_BASE):$(IMG_TAG)
 # BUNDLE_IMG defines the image:tag used for the bundle.
 # You can use it as an arg. (E.g make bundle-build BUNDLE_IMG=<some-registry>/<project-name-bundle>:<tag>)
 BUNDLE_IMG ?= $(IMAGE_TAG_BASE)-bundle:v$(VERSION)
@@ -221,7 +221,7 @@ deploy: prepare ## Deploy controller to the K8s cluster specified in ~/.kube/con
 	$(KUSTOMIZE) build config/default | kubectl apply --namespace $(OPERATOR_NAMESPACE) -f -
 
 .PHONY: undeploy
-undeploy: ## Undeploy controller from the K8s cluster specified in ~/.kube/config. Call with ignore-not-found=true to ignore resource not found errors during deletion.
+undeploy: prepare ## Undeploy controller from the K8s cluster specified in ~/.kube/config. Call with ignore-not-found=true to ignore resource not found errors during deletion.
 	$(KUSTOMIZE) build config/default | kubectl delete --ignore-not-found=$(ignore-not-found) -f -
 
 ## Location to install dependencies to