refactor / tidy up

[Ed Bordin]

• Repeated stuff like git clone / rsync could be converted to bash functions
• The two nextpnr builds could probably be consolidated into one script
• There may still be files in the repo that are no longer used and could be deleted
• The stuff in build_setup copying python files to _packages might be better as a separate "compile_x"/"install_x" script that can be disabled during dev.
• The two CI steps to download artifact then copy it into place could be one step if we use path to download it into place https://docs.microsoft.com/en-us/azure/devops/pipelines/tasks/utility/download-pipeline-artifact?view=azure-devops
• might be worth adding a flag to disable the BBA artifact build/download easily when doing development on something unrelated to nextpnr (currently several locations need to be commented out)

[Ed Bordin]

leaving nextpnr scripts separate for now, think I've addressed the rest of the list here