Makefile: Remove 'clean' prerequisite from build targets

'clean' has been a prerequisite for:

* conformance-%, since the target landed in 0520bdf (hack: add
  conformance tests for hack builds, 2016-05-26, #45).
* release, since the target landed in 04848d1 (Add release build
  process, 2016-05-27, #47).

clean-vm-% target has been a prerequisite for run-% since the target
landed in 9e2e89e (run make target, 2016-09-23, #141).

But there are two problems with including cleaners are prerequisites
of build steps:

* They are inefficient.  Make is good at rebuilding only things that
  need rebuilding, provided you inform it of your dependencies.
  Cleaning before every build will often lead to unnecessary rebuilds
  of up-to-date content.

* They are racy.  For example, a parallel make invocation may process
  several prerequisites at once.  The current 'clean' recipe will
  execute quickly, which is why it hasn't been much of a problem yet,
  but you can see the race by inserting a delay:

    clean:
      sleep 60
      rm -rf _output

  and running:

    $ make -j8 release
    sleep 60
    mkdir -p _output/bin/darwin/
    GOOS=darwin GOARCH=amd64   go build ...
    ...
    mkdir -p _output/release/
    tar czf _output/release/bootkube.tar.gz -C _output bin/linux/bootkube bin/darwin/bootkube bin/linux/checkpoint
    rm -rf _output

  with the other preqrequisites completing first, clean's rm ends up
  removing everything release generated.

In this commit, I've removed the clean prerequisites from the build
targets to resolve the above issues.  I've also updated our
recommended make invocations to add clean as an explicit earlier step.
This ensures folks following our directions get the same behavior as
they would have previously (since our recommendations are all
implicitly -j1 and rm is fast anyway).  I think the extra clean calls
may be overkill in at least some cases, but I'll remove them one by
one in follow-up commits.

.travis.yml

@@ -7,4 +7,5 @@ go:
   - 1.9.x
   - 1.10.x
 script:
-  - make clean release
+  - make clean
+  - make release

Documentation/development.md

@@ -17,20 +17,23 @@ cd $GOPATH/src/github.com/kubernetes-incubator/bootkube
 Then, to build (only Go verson 1.8 is supported now):
 
 ```
-make clean all
+make clean
+make all
 ```
 
 ## Local Development Environments
 
-To easily launch local vagrant development clusters:
+To easily launch local, single-node vagrant development clusters:
 
 ```
-# Launch a single-node cluster
+make clean-vm-single
 make run-single
 ```
 
+You can also launch a multi-node cluster:
+
 ```
-# Launch a multi-node cluster
+make clean-vm-multi
 make run-multi
 ```
 
@@ -39,10 +42,12 @@ Each of these commands will recompile bootkube, then render new assets and provi
 Additionally, if you wish to run upstream Kubernetes conformance tests against these local clusters:
 
 ```
+make clean
 make conformance-single
 ```
 
 ```
+make clean
 make conformance-multi
 ```
 

Makefile

@@ -26,7 +26,6 @@ cross: \
 	_output/bin/linux/s390x/checkpoint
 
 release: \
-	clean \
 	check \
 	_output/release/bootkube.tar.gz \
 
@@ -56,7 +55,7 @@ _output/release/bootkube.tar.gz: _output/bin/linux/bootkube _output/bin/darwin/b
 	tar czf $@ -C _output bin/linux/bootkube bin/darwin/bootkube bin/linux/checkpoint
 
 run-%: GOFLAGS = -i
-run-%: clean-vm-% _output/bin/linux/bootkube _output/bin/$(LOCAL_OS)/bootkube
+run-%: _output/bin/linux/bootkube _output/bin/$(LOCAL_OS)/bootkube
 	@cd hack/$*-node && ./bootkube-up
 	@echo "Bootkube ready"
 
@@ -68,7 +67,7 @@ clean-vm-%:
 	    rm -rf cluster )
 
 #TODO(aaron): Prompt because this is destructive
-conformance-%: clean all
+conformance-%: all
 	@cd hack/$*-node && vagrant destroy -f
 	@cd hack/$*-node && rm -rf cluster
 	@cd hack/$*-node && ./bootkube-up

RELEASING.md

@@ -72,6 +72,7 @@ Or, manually:
 
 ```
 # Vagrant
+make clean
 make conformance-multi
 ```
 
@@ -96,6 +97,7 @@ Or, manually:
 
 ```
 git checkout vX.Y.Z
+make clean
 make release
 PUSH_IMAGE=true ./build/build-image.sh
 ```
@@ -112,6 +114,7 @@ Or, manually:
 
 ```
 git checkout master # Checkpointer releases should only be built from commits reachable by master
+make clean
 make release
 BUILD_IMAGE=checkpoint PUSH_IMAGE=true ./build/build-image.sh
 ```