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, 26 insertions, 0 deletions

diff --git a/target/android_arm64-v8a.rb b/target/android_arm64-v8a.rb
new file mode 100644
index 000000000..70b0f4b97
--- /dev/null
+++ b/target/android_arm64-v8a.rb
@@ -0,0 +1,26 @@
+MRuby::Build.new do |conf|
+
+  # Gets set by the VS command prompts.
+  if ENV['VisualStudioVersion'] || ENV['VSINSTALLDIR']
+    toolchain :visualcpp
+  else
+    toolchain :gcc
+  end
+
+  enable_debug
+
+  # include the default GEMs
+  conf.gembox 'default'
+end
+
+# Requires Android NDK r13 or later.
+MRuby::CrossBuild.new('android-arm64-v8a') do |conf|
+  params = {
+    :arch => 'arm64-v8a',
+    :platform => 'android-24',
+    :toolchain => :clang,
+  }
+  toolchain :android, params
+
+  conf.gembox 'default'
+end