build: fix the default target of Makefile

The current default target of Makefile is "_output/charts".

```
$ make -d
...
Successfully remade target file '/home/sat/go/src/github.com/rook/rook/_output/charts'.
```

It's because Makefile first includes "build/makelib/helm.mk", this file defines
some targets, and the first entry is regarded as the default target.

Signed-off-by: Satoru Takeuchi <satoru.takeuchi@gmail.com>

Makefile

@@ -17,6 +17,7 @@ include build/makelib/helm.mk
 
 .PHONY: all
 all: build
+.DEFAULT_GOAL := all
 
 # ====================================================================================
 # Build Options