Move "-s -w" flags to GOLDFLAGS as an overridable default.

If GOLDFLAGS is set, whether it is empty or not, we should honor it.
Only if the GOLDFLAGS is totally not set, then we use "-s -w"

See Parameter Expansion section in the urls below:

https://stackoverflow.com/questions/3601515/how-to-check-if-a-variable-is-set-in-bash/16753536
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02

Change-Id: I826c34efc63c77f0e3e9677fff30a3eb2219a377

hack/.shellcheck_failures

@@ -27,6 +27,7 @@
 ./hack/cherry_pick_pull.sh
 ./hack/ginkgo-e2e.sh
 ./hack/grab-profiles.sh
+./hack/lib/golang.sh
 ./hack/lib/init.sh
 ./hack/lib/swagger.sh
 ./hack/lib/test.sh

hack/lib/golang.sh

@@ -693,7 +693,7 @@ kube::golang::build_binaries() {
     host_platform=$(kube::golang::host_platform)
 
     local goflags goldflags goasmflags gogcflags
-    goldflags="${GOLDFLAGS:-} -s -w $(kube::version::ldflags)"
+    goldflags="${GOLDFLAGS=-s -w} $(kube::version::ldflags)"
     goasmflags="-trimpath=${KUBE_ROOT}"
     gogcflags="${GOGCFLAGS:-} -trimpath=${KUBE_ROOT}"