Build process updated:

You have to specify `TARGET` to specify a configuration, e.g.

```
rake TARGET=host-debug all test
```

When you port `mruby` to a new configuration:
1. copy an existing configuration under `target` directory
2. modify the new configuration file
3. build using the new configuration
4. send PR if you please

lib/mruby/lockfile.rb

@@ -26,7 +26,7 @@ module MRuby
       end
 
       def instance
-        @instance ||= new("#{MRUBY_CONFIG}.lock")
+        @instance ||= new("#{MRUBY_ROOT}/build/#{MRUBY_TARGET}.lock")
       end
     end