*: No need to 'make clean' before calling 'make conformance-...'

The hack/ content only depends on _output for
_output/bin/${local_os}/bootkube and _output/bin/linux/bootkube.
conformance-% only uses hack/ content, and it depends on all.  'all',
in turn, depends on both _output/bin/$(LOCAL_OS)/bootkube and
_output/bin/linux/bootkube, and each of those fully specifies their
build dependencies.  So there's no need for a separate clean step
before running the conformance-% targets; Make will rebuild anything
that needs rebuilding already.

Author: wking

Documentation/development.md

@@ -42,12 +42,10 @@ 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
 ```
 

RELEASING.md

@@ -72,7 +72,6 @@ Or, manually:
 
 ```
 # Vagrant
-make clean
 make conformance-multi
 ```