This commit mainly accomplish two things:
1. Use makefile to manage dependencies for `occlum build`, which can save lots of time
2. Take dirs `build`, `run` outside from `.occlum`. Remove env var "OCCLUM_INSTANCE_DIR"
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".