Makefile: require KUBECONFIG to be specified for e2e-test target

This Patch adds a check on make target e2e-test, to check if the
KUBECONFIG env var is set before running the test suite.

if not pressent return a valid error message.

Signed-off-by: Carlos Eduardo Arango Gutierrez <carangog@redhat.com>

Makefile

@@ -113,7 +113,8 @@ test:
 	$(GO_CMD) test ./cmd/... ./pkg/...
 
 e2e-test:
-	$(GO_CMD) test -v ./test/e2e/ -args -nfd.repo=$(IMAGE_REPO) -nfd.tag=$(IMAGE_TAG_NAME) -kubeconfig=$(KUBECONFIG) -nfd.e2e-config=$(E2E_TEST_CONFIG) -ginkgo.focus="\[NFD\]"
+	@if [ -z ${KUBECONFIG} ]; then echo "[ERR] KUBECONFIG missing, must be defined";  exit 1; fi
+	$(GO_CMD) test -v ./test/e2e/ -args -nfd.repo=$(IMAGE_REPO) -nfd.tag=$(IMAGE_TAG_NAME) -kubeconfig=$(KUBECONFIG) -nfd.e2e-config=$(E2E_TEST_CONFIG) -ginkgo.focus="\[NFD\]" 
 
 push:
 	$(IMAGE_PUSH_CMD) $(IMAGE_TAG)