build: extract DEFAULT_KUBE_MINOR and add print-var target

Set as default kubernetes version 1.35 in the Makefile to have a central
place to define the costant. The print-var will be used by the GHA in
the next commit to retrieve the version.

Assisted-by: AI
Signed-off-by: Alice Frosi <afrosi@redhat.com>

Author: alicefr

Makefile

@@ -6,7 +6,8 @@ CONTAINER_TOOL ?= podman
 BINK_CLUSTER_NAME ?= e2e
 KUBECONFIG_BINK ?= ./kubeconfig-$(BINK_CLUSTER_NAME)
 ARTIFACTS ?= $(abspath _output/logs)
-BINK_NODE_DISK_IMAGE ?= ghcr.io/bootc-dev/bink/node:v1.35-fedora-44-disk
+DEFAULT_KUBE_MINOR ?= 1.35
+BINK_NODE_DISK_IMAGE ?= ghcr.io/bootc-dev/bink/node:v$(DEFAULT_KUBE_MINOR)-fedora-44-disk
 BINK_LOCAL_REGISTRY_NODE_IMAGE ?= registry.cluster.local:5000/node
 # YEAR defines the year value used for substituting the YEAR placeholder in the boilerplate header.
 YEAR ?= $(shell date +%Y)
@@ -21,6 +22,9 @@ all: build
 
 ##@ General
 
+print-var-%:
+	@echo $($*)
+
 .PHONY: help
 help: ## Display this help.
 	@awk 'BEGIN {FS = ":.*##"; printf "\nUsage:\n  make \033[36m<target>\033[0m\n"} /^[a-zA-Z_0-9-]+:.*?##/ { printf "  \033[36m%-15s\033[0m %s\n", $$1, $$2 } /^##@/ { printf "\n\033[1m%s\033[0m\n", substr($$0, 5) } ' $(MAKEFILE_LIST)