element.py: add configuration for sandbox architecture and os
Description
This allows pulling things built on a different os or architecture.
This essentially backports (and reworks) 1ad35fcd, e5d8c7d8, b0603fb0 (from !969 (merged))
Changes proposed in this merge request:
- add configuration for sandbox architecture and os
- allow builds of 32-bit arch on the corresponding 64-bit arch
This merge request, when approved, will close: #523 (closed)