Add sample config file

and produces build rules.  The build rules are collected by blueprint and

written to a [ninja](http://ninja-build.org) build file.

## Environment Variables Config File

Soong can optionally load environment variables from a pre-specified

configuration file during startup. These environment variables can be used

to control the behavior of the build. For example, these variables can determine

whether remote-execution should be used for the build or not.

The `ANDROID_BUILD_ENVIRONMENT_CONFIG_DIR` environment variable specifies the

directory in which the config file should be searched for. The

`ANDROID_BUILD_ENVIRONMENT_CONFIG` variable determines the name of the config

file to be searched for within the config directory. For example, the following

build comand will load `ENV_VAR_1` and `ENV_VAR_2` environment variables from

the `example_config.json` file inside the `build/soong` directory.

```

ANDROID_BUILD_ENVIRONMENT_CONFIG_DIR=build/soong \

  ANDROID_BUILD_ENVIRONMENT_CONFIG=example_config \

  build/soong/soong_ui.bash

```

## Other documentation

* [Best Practices](docs/best_practices.md)

{

    "env": {

	"ENV_VAR_1": "Value1",

	"ENV_VAR_2": "Value2"

    }

}