Add "untrusted" sections for environment variables defined in Occlum.json. Environment
variable defined in "default" will be shown in libos directly. Environment variable
defined in "untrusted" can be passed from occlum run or PAL layer and can override
the value in "default" and thus is considered "untrusted".
1. All generated, build files are now in a separate build directory;
2. The CLI tool supports three sub-commands: init, build, and run;
3. Refactor tests to use the new tool.