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

1 files changed, 2 insertions, 1 deletions

diff --git a/Rakefile b/Rakefile
index ce3da1d9a..6ccdcd958 100644
--- a/Rakefile
+++ b/Rakefile
@@ -14,7 +14,8 @@ require "mruby-core-ext"
 require "mruby/build"
 
 # load configuration file
-MRUBY_CONFIG = (ENV['MRUBY_CONFIG'] && ENV['MRUBY_CONFIG'] != '') ? ENV['MRUBY_CONFIG'] : "#{MRUBY_ROOT}/build_config.rb"
+MRUBY_TARGET = ENV['MRUBY_TARGET'] || ENV['TARGET'] || "host"
+MRUBY_CONFIG = "#{MRUBY_ROOT}/target/#{MRUBY_TARGET}.rb"
 load MRUBY_CONFIG
 
 # load basic rules