When installers are used, sgx-sdk of Occlum version could have conflicts with official sgx-sdk.
This patch will make sure Occlum command use Occlum specific sgx-sdk.
Also add symbolic links to PAL library of hardware mode when installing.
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"