Support non-bash based script bootstrap #3012
Description
Right now, the stage1 script bootstrap requires bash. It has a variety of bashisms or non-posix constructs that prevent it from working in other shells.
Bash isn't bad, it is just a heavier weight shell than e.g. dash (default on ubuntu/debian).
Looking at the code most changes are minor. The major change is the usage of arrays -- posix doesn't actually specify arrays.
Related: #2500