build/release-images.sh: remove possible duplicate targets

Previously, cmd/kubectl is duplicate as following:
$ make V=1 quick-release-images
...
+++ [0310 11:30:50] Building go targets for linux/amd64:
    cmd/kube-apiserver
    cmd/kube-controller-manager
    cmd/kube-scheduler
    cmd/kube-proxy
    vendor/github.com/onsi/ginkgo/ginkgo
    test/e2e/e2e.test
    cluster/images/conformance/go-runner
    cmd/kubectl
    cmd/kubelet
    cmd/kubectl

Co-Authored-By: Benjamin Elder <[email protected]>
Suggested-by: Davanum Srinivas <[email protected]>
Signed-off-by: Li Zhijian <[email protected]>

Author: zhijianli88

build/release-images.sh

@@ -31,7 +31,9 @@ if [[ "${KUBE_BUILD_CONFORMANCE}" =~ [yY] ]]; then
 fi
 
 # TODO(dims): Remove this when we get rid of hyperkube image
-CMD_TARGETS="${CMD_TARGETS} cmd/kubelet cmd/kubectl"
+if [[ "${KUBE_BUILD_HYPERKUBE}" =~ [yY] ]]; then
+    CMD_TARGETS=$(echo "${CMD_TARGETS} cmd/kubelet cmd/kubectl" | tr ' ' '\n' | sort | uniq | tr '\n' ' ')
+fi
 
 kube::build::verify_prereqs
 kube::build::build_image