Post-Build Cleanup, etc.

scripts/builder.py...

GenReleaseReadme:

• combine with ReleaseReadmeUpdater
• generates README_<profile>.md
• README_alpine.md is a symlink to README.md
• don't crash when README doesn't preexist
• append image list to README if no list found to replace

MakeAMIs:

• collect all artifact IDs and report after all builds
• don't update releases/readme

PruneAMIs:

• defaults to pretend mode, unless --no-pretend
• improve readability

UpdateReleases:

• replace code with what was RefreshReleases