build: remove FileRule on config.mk and fix CUDA config
build: remove FileRule on config.mk and fix CUDA config

* remove FileRule on config.mk, we always override it.
  Due to the following workflow is quite common:
    * user issue `Pkg.build("MXNet")`

    * then, build failed because the `CUDA_HOME` not set properly.

    * if the `FileRule` find `config.mk`,
      re-issuing `Pkg.build` will not change the `config.mk`.
      User need to delete/modified `config.mk` manually

* fix CUDA config
    * the build_steps didn't being unfolded correctly.

    * consider following cases:

```julia
@build_steps begin
  if true
    `1`
    `2`
    `3`
    if true
      `42`
    end
  end
end
BinDeps.SynchronousStepCollection(Any[`42`], "", "")
```

vs

```julia
@build_steps begin
  if true
    @build_steps begin
      `1`
      `2`
      `3`
      if true
        `42`
      end
    end  # inner build_steps
  end  # if
end
BinDeps.SynchronousStepCollection(Any[`1`, `2`, `3`, `42`], "", "")
```

mbaijal/incubator-mxnet,szha/mxnet,zhreshold/mxnet,zhreshold/mxnet,zhreshold/mxnet,ptrendx/mxnet,ptrendx/mxnet,yajiedesign/mxnet,larroy/mxnet,leezu/mxnet,szha/mxnet,reminisce/mxnet,ForkedReposBak/mxnet,ptrendx/mxnet,yajiedesign/mxnet,dmlc/mxnet,eric-haibin-lin/mxnet,ForkedReposBak/mxnet,tlby/mxnet,ForkedReposBak/mxnet,tlby/mxnet,leezu/mxnet,ptrendx/mxnet,dmlc/mxnet,zhreshold/mxnet,dmlc/mxnet,eric-haibin-lin/mxnet,zhreshold/mxnet,apache/incubator-mxnet,tlby/mxnet,szha/mxnet,szha/mxnet,dmlc/mxnet,larroy/mxnet,dmlc/mxnet,eric-haibin-lin/mxnet,reminisce/mxnet,dmlc/mxnet,reminisce/mxnet,DickJC123/mxnet,sxjscience/mxnet,tlby/mxnet,ForkedReposBak/mxnet,mbaijal/incubator-mxnet,eric-haibin-lin/mxnet,reminisce/mxnet,ForkedReposBak/mxnet,yajiedesign/mxnet,sxjscience/mxnet,eric-haibin-lin/mxnet,zhreshold/mxnet,zhreshold/mxnet,ptrendx/mxnet,sxjscience/mxnet,reminisce/mxnet,larroy/mxnet,yajiedesign/mxnet,mbaijal/incubator-mxnet,eric-haibin-lin/mxnet,zhreshold/mxnet,mbaijal/incubator-mxnet,szha/mxnet,mbaijal/incubator-mxnet,eric-haibin-lin/mxnet,yajiedesign/mxnet,szha/mxnet,szha/mxnet,leezu/mxnet,sxjscience/mxnet,szha/mxnet,dmlc/mxnet,eric-haibin-lin/mxnet,mbaijal/incubator-mxnet,dmlc/mxnet,larroy/mxnet,reminisce/mxnet,leezu/mxnet,mbaijal/incubator-mxnet,sxjscience/mxnet,ForkedReposBak/mxnet,leezu/mxnet,larroy/mxnet,reminisce/mxnet,tlby/mxnet,larroy/mxnet,apache/incubator-mxnet,ptrendx/mxnet,tlby/mxnet,sxjscience/mxnet,tlby/mxnet,DickJC123/mxnet,ptrendx/mxnet,ForkedReposBak/mxnet,ptrendx/mxnet,larroy/mxnet,ptrendx/mxnet,DickJC123/mxnet,yajiedesign/mxnet,sxjscience/mxnet,tlby/mxnet,sxjscience/mxnet,yajiedesign/mxnet,reminisce/mxnet,mbaijal/incubator-mxnet,szha/mxnet,dmlc/mxnet,apache/incubator-mxnet,apache/incubator-mxnet,leezu/mxnet,reminisce/mxnet,tlby/mxnet,larroy/mxnet,leezu/mxnet,ForkedReposBak/mxnet,reminisce/mxnet,yajiedesign/mxnet,ptrendx/mxnet,zhreshold/mxnet,yajiedesign/mxnet,sxjscience/mxnet,DickJC123/mxnet,leezu/mxnet,mbaijal/incubator-mxnet,leezu/mxnet,tlby/mxnet,eric-haibin-lin/mxnet,ForkedReposBak/mxnet,larroy/mxnet