common: Fix HomeDir detection

I am working on porting geth to [Ubuntu Core](https://developer.ubuntu.com/en/snappy/https://developer.ubuntu.com/en/snappy/). I am testing geth on a Raspberry PI and for Ubuntu Core the $HOME directory is unique for each application. See [here](https://developer.ubuntu.com/en/snappy/guides/filesystem-layout) for more information of their filesystem layout.

For some reason in Go `usr.HomeDir` returns a different value than `$HOME` in Ubuntu Core.

Adding this at the end of `HomeDir()`
```go
fmt.Printf("at HomeDir, user.HomeDir = %s and $HOME is %s\n", usr.HomeDir, os.Getenv("HOME"))
```

gives the following output

```
at HomeDir, user.HomeDir = /home/ubuntu and $HOME is /home/ubuntu/apps/geth.sideload/IJcODREBYbHO
```

With this commit, I propose giving precedence to the `$HOME` environment variable as is also suggested by the [homedir](https://github.com/mitchellh/go-homedir/blob/master/homedir.go) project.