Integrated batch scripts into build.ps1
build.ps1: Use PowerShell to run things instead of VSPrompt.
build.ps1: Script now stops building new items if any item has failed to build (fail-fast).

- Currently building items are still allowed to run to completion.
- Now also prints a summary at the end.
- Miscellaneous prettification.
build.ps1: Ignore error code 2 returned by openssl's clean step because failing to clean build outputs that don't exist is not a failure.