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

target/host-debug.rb

@@ -11,7 +11,7 @@ MRuby::Build.new('host-debug') do |conf|
   enable_debug
 
   # include the default GEMs
-  conf.gembox 'default'
+  conf.gembox 'full-core'
 
   # C compiler settings
   conf.cc.defines = %w(MRB_ENABLE_DEBUG_HOOK)

target/host.rb

@@ -20,7 +20,7 @@ MRuby::Build.new do |conf|
   # conf.gem :git => 'git@github.com:mattn/mruby-onig-regexp.git', :branch => 'master', :options => '-v'
 
   # include the GEM box
-  conf.gembox 'default'
+  conf.gembox 'full-core'
 
   # C compiler settings
   # conf.cc do |cc|