Add a script to initialize external repos manually

Here is the full sequence implemented to handle external package.

1. if the source is directly git cloned. CMake directly uses git submodule command assuming internet is available in this case.
2. if the build is using tarball and there is internet. CMake handles git init and fetch the source based on submodule_commit_hash_records
3. if git fetch failed because of no internet or other reasons. Users can execute initialize_external_repos.sh to manually initialize all the files of external repos. If needed, transfer "external" folder from a machine with internet to a machine without.

Hopefully this covers all use cases.

I also tried first "initialize_external_repos.sh" to initialize all external packages and then use configure. There is no conflict with the pre-initialized devxlib. If we can add the same logic 1,2,3 in configure, we can fully avoid storing source code in archive.

closes #331 (closed)