You can build this. There's different strategies, but the least work option would be a minimal install of your OS of choice with your choice of REPL installed a

You can build this. There's different strategies, but the least work option would be a minimal install of your OS of choice with your choice of REPL installed as /bin/init (or you can usually set init= as a kernel parameter) You'd probably need to tune your chosen kernel to hit 500ms, but it's doable. Harder if you include the time in system firmware though.