enhancement: place extgen.sh in separate directory and pass in environment variables

[Lslightly]

Repositories are cloned in specific commit without extgen.sh often. So it's better to pass REPO_DIR and OUTPUT_DIR environment variable to the extgen.sh script.

• REPO_DIR is the repository directory

• OUTPUT_DIR is the extgen directory

• PROJROOT is the root directory of the project

• DB_EXT_DIR is the directory to store external predicate database

• genScript should be modified to relative path to project root.

[Lslightly]

build.sh also need to be placed in separate directory and pass in environment variables