makefile: check for $(CILIUM_CLI) dependency

[msune]

Prior to this commit executing any target that invoked $(CILIUM_CLI) silently failed (as of $?==0), e.g. kind-install-cilium.

This commit adds check_deps target in the main Makefile, and adds it as a target dep for all targets using $(CILIUM_CLI).

See for instance:

~/dev/cilium$ make kind-install-cilium
kind is ready
  INSTALL cilium
# cilium-cli doesn't support idempotent installs, so we uninstall and
# reinstall here. https://github.com/cilium/cilium-cli/issues/205
# cilium-cli's --wait flag doesn't work, so we just force it to run
# in the background instead and wait for the resources to be available.
# https://github.com/cilium/cilium-cli/issues/1070
cilium install \
	--chart-directory=/home/msuneclo/dev/cilium/install/kubernetes/cilium \
	--helm-values=/home/msuneclo/dev/cilium/contrib/testing/kind-common.yaml --helm-values=/home/msuneclo/dev/cilium/contrib/testing/kind-values.yaml \
	--version= \
	>/dev/null 2>&1 &
~/dev/cilium$ echo $?
0
~/dev/cilium$ cilium --help
cilium: command not found

Issue

No issue, but I can create one if necessary.

kubectl and other deps are invoked in foreground, so they fail as expected. I could add them to check_deps though, to have cleaner messages. Please advise.

Release notes

Not sure if worth. Please advise.

[joestringer]

/test