Prototype VCS CI running on AWS

This is a prototype of a VCS CI running on AWS. .github/workflows/aws.yml was provided by @zarubaf and I have not modified it in any meaningful way. The buildspec.yml file has been substantially changed and this is where reviewers should spend most of their time.

Other changes include updates to bin/ci_check to ensure it generates a valid and accurate return code and vcs.mk to minimize the size of the logfile generated.

This PR may not survive a review, but I am asking for your review now because there will be core-specific buildspecs in future PRs.